If you just mean the outside plastic surface of the taillights, I've had good results using a plastic polish like Novus #2 (or for severe cases, Novus #3 then Novus #2). Plast-X is easier to find but doesn't work quite as well.
I am talking about the part behind the light bulb that the light reflects off of and out the lens. I know my car isnt in that good a shape yet, but it bugs me when i see a nice old car going down the road and the lights in the back are not the same brightness or just really dull.
I did this recently on a spider I just acquired. I assume you mean the chrom plated plastic inside the lens. I used a light rubbing compund and then a chrome polish. Experimentation works; try different combinations of polishes and compounds until you find one that you are satisfied with. Some stains wouldn't polish out, but overall they are now very shiny, vs the dull gleam they had before.
On the exterior (colored) lens, Novus is an excellent choice. If you go to the Novus website, they will sell you a small kit of 3 polish grades and cloths for around $12-$15, including delivery. It took about 2 days to get it. Also works wonders on convertible top plastic rear windows.