3DPrintUK has made an excellent tutorial on things to avoid when making a CAD file fit for exporting to 3D Printing.
I’d add a few things to their list of suggestions myself:
1.) Know the minimum thicknesses for details and supporting walls you can get away with on the material in which your printing. This varies by 3D printing material, but Shapeways has a good starting list for 3D printing tolerances.
2.) Keep in mind how the model will be ultimately used. Just because a piece is thick enough to survive 3D printing doesn’t mean it’s thick enough to survive casting or wear.
3.) If the model is destined to be polished or finished in some way, think about how reachable any surface details are going to be. If it helps, it might be a good idea to measure your polishing tools.