I can get them clear again but they don't stay like that forever. I find a coat of wax helps a lot.
Yep if you don't put some sort of UV coating on the lens after polishing it, it will fog over again. Typically these restoration kits involve stripping/polishing off the protective UV coating on the lens. Once the UV coating is gone the lens yellows or fogs over. Only way to fix is to either replace the lens/light housing or use a sealant. Down side to the sealant is one typically has to re-polish and reseal the lens once or twice every year or two.
Currently I use Meguiar's PlastX and Blue Magic Headlight Lens Sealer, have to use it once or twice a year on a car that is parked outsided 24/7. Others have raved about the 3M type kits that use a polisher on a drill.