start with 1200-1500 grit sandpaper and wet sand the lense, then move to higher grit around 2000 and wet sand again. Finish off with some 3M rubbing compound. They will look like new. You can use a lower grit when starting to make quicker work of the sandpaper, but if you go too deep in the lense it will make the scratches a lil harder to remove.
Wetsanding works great, but PlastX also works very well.
The only downside to PlastX is that you need to keep re-applying it, or the lenses will start to fade in time (because they aren't sealed after using abraisives).
Always finish up with a buffing glaze of some sort that will seal the plastic.