What material should the bag be made of and how thick should it be?

Polyethylene is the material of choice. This poly material is cost effective, easy to work with, and provides good protection for the garment. Polyethylene can be used as either a dust cover or as retail packaging. Higher quality bags are generally 1.5 mils thick and "dust cover" types of bag typically range from 1-1.25 mils.

Other materials, such as Polyolefin, Polypropylene, and co-extruded materials, are available. While they can look very nice, they are more expensive and can be difficult to work with.