Wolfram posted videos of its Visualization Virtual Workshop 2013 recently. One of the videos shows how to create 3-D objects suitable for printing on a 3-D printer. The video Scan, Convert, and Print: Playing with 3D Objects in Mathematica show the details of creating shapes from wire meshes, formulas, and scans with the proper thickness, surface checking, and generating of the proper files compatible with 3-D printer (.stl). A useful tool for inspecting your 3-D print files is MeshLab. It's open source. The presentation files are available from the first link above.