Bevel edges to allow for a realistic play of light across the plane change.
It helps to occasionally view your model in grey shaded mode with a glossy material - the specular highlights reveal problems in the curvature of the surface.
If the software does already have this feature, install a script that will let you hotkey the “spin quads/faces/polys”. This lets you quickly adjust the topology, rotating an internal edge of a quad so that it creates a nice continuous line with nearby quads.
Make a separate sphere for an eyeball, and model the lids around that (as you would in traditional sculpture).
Quick tips
For the most part, work from general to specific. Start with a primitive such as a box, and then extrude and insert various parts of the mesh to create the overall form. The goal is to create a model that has sufficient resolution for hi-res renders but is light enough so that editing the model doesn't become overwhelming. This is done by creating a low-res base mesh of the entire model that only has the minimum number of faces to define the form, making it easy to edit, but with enough detail in the right places so that a smoothing operation automatically up-rezzes the entire model. This is done by only putting detail where it is needed, using fencing and good edge flow. This prevents you from having to model the high-res mesh by hand. Your model should be a low-res mesh that indicates to the smoothing operation how to interpolate your mesh to create the high-res one.
Create good topology or edge flow that mimics the structure of the reference object, whether it's a human, a car, etc. Plan the topology of the model in advance by drawing edge loops on the reference images by hand in photoshop. Use quads whenever possible, avoiding triangles and poles except when necessary. Use fencing so that the forms are preserved after smoothing operations are applied.
Use instances whenever possible - never model the same object twice.
Use mirroring whenever possible - never make the same component/subobject adjustment twice.
Name your objects in an organized fashion. Use letters to designate a different variation of a thing (hat_A_001, hat_B_001) and numbers for the versions of files and increments of instances of an object (file_A_001.ma, file_A_002.ma, hat_A_001, hat_A_002). Always add an increment number to every object in anticipation of it being instanced.
Organize objects into groups and layers as appropriate. Groups are for organizing the transformation of objects. Layers are for organizing the visibility of objects.
Create a custom hotkey for any operation you find yourself doing repeatedly (toggling component/object mode, split poly, adding edge loops/rings, spinning quads, welding/merging verts).
Use snapping as a construction aid.
Use the "Wireframe on Shaded" shading mode to view the wireframe and surface together while modeling the object.
Press g to repeat the last operation.
Topology
Model using mostly quads (quadrangles, or four-sided polgyon faces). Only use the following polygon types when necessary, to control edge flow:
triangles
3-edge poles
5-edge poles
Automated tools that create edge loops will not pass through triangles - they will only pass through quads.
Using the automated tool for creating edge loops is a good test that the topology is good. If the loop breaks at some point, then there's bad edge flow that should be fixed.
It's ok to draw edges when you first start creating a new shape, but soon after that you should take advantage of automated tools for creating edge loops, etc. Otherwise, drawing loops is far too slow.
In general, model using good topology, or polygon flow that mimics real muscles. For example, the muscles around a real human mouth are circular and concentric, so a topology that is circular and concentric around the mouth will create the most realistic deformations when the mouth is animated.
One helpful technique for planning the topology of the model is to draw edge loops by hand on your reference images in photoshop.
Fencing
Fencing is the addition of edges on either side of an existing edge that help define how much to smooth the original edge when a smoothing operation is applied. For example, adding "fencing" edges very close to either side of a box will maintain that sharp edge of the box when smoothed, but placing the fencing farther away from the edge of the box will make it more rounded when smoothed.
The placement of fencing also affects how the model deforms. Use the “rule of three” around joints to get good deformations - one edge ring around the joint itself, and one edge ring on either side of the joint, close to it. This rule should be used at joints such as shoulders, elbows, hips, ankles, etc. It helps maintain volume when the joint is rotated.
Poles
A pole or star is the intersection of multiple edges at a single vertex. The poles created by adjacent quads have 4 edges, which is fine. A 2-edge pole is considered bad topology, as are poles that have 6 or more edges. However, N-poles (where 3 edges converge to a point) and E-poles (where 5 edges converge to a point) are sometimes necessary because they allow the edge flow to change direction. N and E poles should be used sparingly, however.
A quick way to fix poles is to select two adjacent faces around the star point at a time and merge them. Do that all the way around the star. Then go back and spin quads on all those new faces until the topology looks good.
Don’t put poles in areas that deform.
Here's an in-depth discussion of poles on Topowiki
Transitioning between resolutions
This post on the Second Life forums seems to be a great guide to creating clean topology
In section 9 of the link above, there’s a diagram of how to transition from a flow of 4 faces down to 2, and 6 faces down to 2. The key to this transition is the “diamond-shaped” quad, which can be modeled to look exactly like a triangle, or you could pull the extra point out of the triangle shape to create the diamond shape. This technique allows you to put higher resolution in the model in places that need it, and then transition to lower res geo for parts that won’t deform much.
If you already have the dense mesh and realize that you need to lower the res of part of it (such as when the edges converge in a tapered end of the model), then figure out where you want the topology to downres to just 2 faces. The first diamond will go directly above that, creating 3 faces side-by-side. Then the next triangle will go directly above that, creating 5 faces side-by-side. Finally, directly above that will be your hi-res geo, with 6 faces side-by-side, completing the transition. It’s tricky to visualize how to create the transition, but with practice it is possible to manually move this transition around on the mesh, undo it, etc, by carefully adding edges with the split poly tool, deleting edges, and spinning edges. Use the top and bottom boundaries as guides, knowing that the six faces above have to flow smoothly down to the 2 faces below, and create those paths as necessary. At some point, you’ll be able to blow away several columns of edges near the bottom of the transition.
Actually, I was able to do the 6-face to 2-face transition without having to spin quads or do too much other wrangling of geometry by working from the top down - something like draw the sides of the top triangle, which start one edge on either side of the center edge. Then in the next row down, draw the sides of the bottom triangle, starting two edges on either side of the center edge. Then delete a bunch of vertical edges to create the necessary flow.
Cleanup
Here's some tips on how to cleanup bad topology:
If there are open edges that sit on top of each other, it might help to delete one of them by deleting a face. Then you can see what's going on in order to make the fix, and it should be easy to fill in that deleted face when you're done.
Make sure that verts are actually connected to each other by an edge before trying to weld them.
Sometimes it helps to delete some faces and replace them by extruding surrounding edges and welding the new verts to the old ones to "patch" the hole.
Temporarily pulling apart edges and faces and then undoing your change can help reveal what the problem is.
You can select faces to help see where the problem areas are, revealing overlapping edges, faces, etc as indicated by an unexpected interruption in the selection highlighting.
Some 3D packages don't allow you to weld together faces that would become enclosed inside the object, so delete the end cap faces of two parts that you're trying to weld together before attempting to weld/merge their verts.
Interpenetrations
When fixing interpenetration problems (such as the result of extruding 3D text where interpenetrations occur in the notches where letters meet), first decide on a side of the area that you’ll keep the way it is - this is the side that has the shape you want to preserve. Then pull apart the two penetrating parts on the other side so there’s a gap between them and it’s clear which part is which. Delete faces that would end up being inside the object when you bring the parts together. Then bring the parts together by moving their verts to be coincident where they need to be. Model the shape that you need by eye, and then select the verts in that entire area and merge/weld the verts.
Selections
For hopefully rare situations where you can’t use the Loop and Ring tools to make complex edge selections, you can manually make the selection by first selecting the complex edges and whatever edges surround that area, then ctrl+alt select the surrounding edges you don’t want in order to de-select them. Then work your way down to selecting the simpler edges.
Scale
Scale is a multiplier - it should only be used to multiply the dimensions of a model that’s already been modeled, it should not be used as a modeling technique. Instead, when modeling, edit the model’s dimensions by moving verts/edges/faces in component/sub-object mode.
For example, if you’re modeling a chair, you can scale an instance of the chair model that you might be using for set dressing in a scene to make it say, 10% bigger (although ideally all models would be modeled to the correct scale in the first place), but you shouldn’t scale the chair (in object mode) while you’re modeling that asset. If you did scale a model while you’re creating it, then modifiers applied to the object such as extrude, chamfer, etc could have strange, non-proportional results because they act based on the scale multipliers in x, y, and z.
Most software has an option to specify the actual dimensions of a primitive upon creation, which is the proper way to specify the size of the object initially. For example, in maya, when you create a cube, in the channel box, the Scale X, Scale Y, and Scale Z transform channels are multipliers only, and should not be used while modeling. Instead, use the Width, Height, and Depth INPUTS for the polyCube.
Similarly, in 3ds Max, when you create a Box primitive, you can use Length, Width, and Height in the Parameters rollout to specify the actual dimensions of the box upon creation, and the same parameters are later available in the Modify panel as well. Do not use the scale tool, or the X, Y, Z values when you right-click the scale tool, or the X, Y, Z values shown at the bottom of the screen.
After the initial creation of the object, you should edit its dimensions by moving verts/edges/faces in component/sub-object mode. You can also use the scale tool while you’re modeling an object, but only use it while in component/sub-object mode, never while in object mode. (This scaling technique is essentially a shortcut to moving a collection of verts/edges/faces together in a specific way.)
Mirroring
Mirror your geometry so that you can just model the character’s left side and the right side will automatically become its mirror.
Using a digital sculpting program such as ZBrush is a great way to quickly model detail, but the mesh will usually need to be retopologized in order to make it production-ready for rigging and animation.
When sculpting, simplify the shapes into flat planes to start, as if the rough model were made of polygons in the computer.
Sculpey
Sculpey III
Bake in a conventional home oven at 275 degrees fahrenheit for 15 minutes per quarter inch of thickness.
Premo!
Bake in a conventional home oven at 275 degrees fahrenheit for 30 minutes per quarter inch of thickness.