A sampling of some of the plugins which can make positioning stuff a bit easier...
Smustard - Quickly change the Camera setting, via 3 options, to obtain a 2 point perspective.
Didier Bur - Align selection (groups and components) to an alignment line.
Didier Bur - Align selection in 3D.
Jim Foltz - Arrow buttons move the selected Group or Component in the direction of the arrow (does not work on geometry.)
Smustard - Nudge (move) objects via the ARROW keys small or large distances.
Render Plus - has several tools for laying out, editing and manipulating 3D components, groups and bits of geometry: there is Place Adjacent, and Aim Component and Polyline wizards and more. The Move wizard is demonstrated below. Windows
Tip: After clicking on a nudge direction on the wizard, use the spacebar to repeat action instead of repeated clicks on the wizard buttons..
To jumpstart plugin development, one of the first sample plugins offered by the SketchUp team back in version 4 was bezier curve. It's not only good for freeform polylines but it may be useful for tracing out profiles on imported images.
Bezier lives in the Draw menu once the plugin is installed.
The editing option appears in the context menu when the curve is selected.
Tips:
The editing option can be made into a keyboard shortcut. Select a bezier curve then go to Window > Preferences > Shortcuts to find this context menu item in the list.
A 3D navigator allows you to reposition the camera with the left hand while using another SketchUp tool with the right hand.
Editing the plugin. The default number of segments in the bezier plugin is 20. To change the number, the plugin code needs to be edited as described below.
The plugins downloaded from the Extension Warehouse are RBZ files. Replace the RBZ extension with ZIP, and unzip the file. The ZIP contains su_bezier folder and su_bezier.rb. Open the su_bezier folder. Inside that folder is bezier.rb.
Open bezier.rb file in a plain text editor, like Notepad (in Windows go to Start > All programs > Accessories) or (better) use Notepad ++:
Change the number at the end of line 199 (Notepad++ has a search tool to find all instances of '20' in the code in case the line number changes). Add a note to yourself about the edit by prefacing the note with #. Save the file. Replace the modified bezier.rb with the new version in SketchUp's Plugins directory. Restart SketchUp and try using Bezier Curve.
If coding isn't your thing, try BezierSpline below.
Fredo6 - Bezier curve-making "on steriods." It maybe useful when drawing profiles around some images. It comes with a PDF manual.
thomthom - Custom edge drawing tools
Terry Ross - Draw a variety of spirals, curves, and tapered shapes (tapered extrusions.)
Fredo6 - It is a Suite of Tools which draw on curved surfaces, with lines, various shapes, offset and Freehand / Polyline. It provides an inference mechanism, a Contour Editor and a dedicated Eraser.