Once a model has been scaled for printing, check all thicknesses and the size of small details (remove or enlarge as appropriate). Models should have walls with a minimum thickness of 0.8 - 2mm (depending on the size and support).
Consider the structural integrity of printed models, don't print trees as solid spheres on thin trunks.
Ensure that all model surfaces are fully closed, ie watertight.
Files should be in binary STL format for single part monochrome models, multiple STL files (with the same origin) for multiple materials and/or colours.
Other, full colour printers might use VRML.