The below are some considerations to keep in mind when designing a part you want to be 3D printed. There are many factors which impact the success of a printed part (technology, material, orientation etc.), all of which it's good to keep in mind when you're designing your part.
Sadly, 3D printers aren't magic machines that can churn out any file you throw at them and have it be an amazing quality part.
Deciding what machine and material you want to use ahead of time can help you to design around the limitations of that machine. For example, if you want a really strong L shaped bracket and want to use FDM, consider adding a rib or a fillet to the inside corner and orienting the part so that the print nozzle lays material in a continuous line in that area. You could also consider printing in a stronger material, or adding carbon fibre to that area.
A small amount of thought and planning goes a long way to achieving the best results from 3D Printing, and means you'll have less work to do on your part after it's printed!
Rib: A rib is a special type of extruded feature that adds material of a specified thickness in a specified direction between the contour and an existing part. Ribs are fundamental features for enhancing the structural integrity of your 3D printed parts. Connecting the part to a wall strengthens the construction. It also supports the part from the bottom surface.
Chamfer: A chamfer is a beveled edge between two (near) perpendicular surfaces in the model. Using chamfers (often 45°) as a transition instead of sharp corners reduces stress and increases the strength of the connection.
Gusset: A gusset is a small triangular extra surface between extruded features and the bottom surface (or wall, when used for horizontal features). Gussets help to support features with more surface contact like a rib. However, they do not connect to another feature or wall, but it tapers off as the Z height increases.
Wall thickness: Generally, designs with a uniform and consistent wall thickness decrease the chance of warping or shrinkage. A consistent wall thickness will improve the overall stability of the printed part, and it avoids weak points in the design and extrusion changes.
Fillet: A fillet is a rounded corner between two surfaces. Similar to chamfers, fillets reduce stress in areas where two surfaces meet and strengthen the connection. Additionally, the smooth transition will often help to improve the visual quality of the print.
Rounded corners: Using rounded corners instead of sharp (90°) transitions help distribute the stress in the material more evenly throughout the model. This enhances the overall print quality and reduces the risk of weak points or fractures. Sharp corners can cause sudden changes in speed, acceleration, and the extrusion rate, which leads to visual quality issues such as ringing / ghosting.