Make the garage greater than standard in all dimensions to allow extra room for cars. This is especially true with the depth of the garage - a 30' deep garage will leave a very nice space for a workbench and area to work on the cars. Use single-wide garage doors, as these allow you to space out the cars a bit (ie, greater than the 10' spacing common in garages), and usually gives a nicer appearance from outside.
As others said, design in *plenty* of electrical capacity! Several 30A, GFCI plugs on each wall. Use plenty of lighting. If possible, plan the space for the workbench and include extra plugs and lights there.
Add a sink! A small bathroom one can be used for this and look much nicer than your traditional laundry tub - though much less functional, of course. Depends on how the sink would be used.
Drywall! Gotta have it fully finished.
Pay special attention to the surface people will see when you look up in the garage. Is it sheetrock? Exposed rafters? If exposed structure, I recommend painting it white to reflect light.
Tile floor!