What grit you use depends on how bad the pitting and scratches are. I've only done it a couple times myself. Start out with 400-800 and work your way up to around 2000 (higher if you can get it). As with just about any sanding your main purpose with the lower grits is to level everything out, then you move on to the next grit to remove any grooves/scratches left by the more abrasive paper. You're done when the lens looks very lightly frosted, if you put a little water on it it should look clear.
After wet sanding the living hell out of the lenses, you need to buff it. Here's where a bench grinder with a couple buffing wheels comes in VERY handy. You can do it by hand, but it's a PITA. Get a couple different grades of buffing compound (medium, fine, very fine). Apply the compound to the wheel and start buffing away. Keep a firm grip on the lens and keep moving it around in small circles. I found that rotating the piece 90º after each pass helps remove any hairline scratches the quickest. Work though the different grades of compound. <--- You're best off using a fresh buffing wheel for each grade compound. Think about trying to sand something with 3000 grit paste while using a sanding block with 200 grit sandpaper behind it.... no it's not that bad, but you'll get better results faster.
Clean the lens.
At this point your results are going to vary a little depending on how well you worked the lens at every stage. Some people can get it looking almost new (if you look close you can still tell, but barely), others end up with it still looking fuzzy. If it still looks frosted (it should be VERY light after all this work if it does) you can use a polish (wax) to clear it up.... Most polishes aren't intended to stand up to the heat and abrasion which these lenses will see though and as such will require more attention than others.
I have heard that it's possible to resurface the lens at this point using extreme heat, but most don't have the tools necissary. I wouldn't recommend trying it with a propane torch (works great on plastic though) because the glass is less forgiving when heated unevenly or too quickly. The goal with this method is to do away with polishes (which just hide imperfections) by heating the very surface of the glass until it turns liquid and smooths itself out. Fortunately, our fog lenses aren't tempered glass... you heat that unevenly and it practically explodes. Still if you're thinking of trying this be VERY VERY CAREFUL!! Wear a thick coat, leather gloves and at least eye protection. When the non-tempered stuff decides it's had enough it's not pretty either (ask me how I know).
As for protective measures, alot of dealers, window tint shops and auto body shops now offer 3M's clear film (the clear bra stuff) for headlights, foglights, and the front face of rearview mirrors. I've even seen it on the lower portion (about 10") of windshields on several trucks and one Evo in my area. The stuff sold today is supposed to age MUCH better, absorb and distribute impact better, and holds up better under higher temps. I suspect that's your best bet, but for the best adhesion the lens needs to be nice and smooth. Scratches and pits will let air and other elements under the film.
