Sandpaper is only needed for a 1st-time, bad state.
Touch up, if you waited too long you can use rubbing compound with plenty of water. If using a motorized tool, move it around a lot so you don't build up heat in one spot.
Next use plastic polish, or if you have a motorized tool and move it around a lot, even toothpaste can work.
Finish with wax or wax-type sealant. I do not recommend a spray on paint-like sealant. It will last a bit longer but still need done again then you have to strip all the sealant off which is more work in the long run.
Periodally touch up the finish. For example when washing the vehicle put a few dabs of toothpaste on a rag and wipe the lenses down.
If parked for several hours a day exposed to sunlight, try to park in a direction with the headlights facing away from the sun. This can also make your wipers last longer.