The stuff I've seen most people use are the white or pink Styrofoam boards like you'd find in the home improvement stores (the foam "Insulating Sheathing" boards - people argue over which color should be used, but, personally, I'd go with the white, as to my knowledge it would be better for relieving pressure points), neoprene mats (like yoga mats), and polyethylene mats (GlassCages, for example, uses 3/4" inch white styrofoam by default, with an optional polyethylene mat upgrade).
The ideal thickness of the material depends on how big the tank is, but, for most tanks, 3/4" would likely suffice.