The local store had the polish for headlight lens restoring on sale for $4.99 (reg $7.99). I've seen this cost anywheres from 30 to 80 bucks depending where you look. So for under 10 bucks including the sandpaper and 30 minutes, I tried it myself.
My wifes 2004 car, with about 250,000kms now is certainly showing its age, especially the headlight lens cover.
Basically its no different than wax, apply it then buff it off in circular motions. For extreme cases it says to use 2000grit wet sandpaper, which in my case we had to do. While it didnt come out perfect, after 3 or 4 times, I'd have to say its a big improvement and should increase her night time visibility. I'm sure if I continued I could get it better still.
First picture shows the right hand side done, and the left hand side not done.
2nd pic: close up of the left hand side not done
3rd pic: close up of the left hand side partially done (so you can see the difference it makes).