I know this is probably not what you want to hear but just have them professionally done. There is a guy here in Vegas that does them for $50 a pair to your desired darkness. After you purchase Niteshades, clear, and factor in your labor time, $50 is dirt cheap and you'd probably be saving money. Mine look like glass and the light still shines through nicely.
I did my F150 myself, using VHT Niteshades and Dupli-color clear. They came out pretty good with several wetsanding sessions and lots of polishing but after 1 summer, the 3rd brake light started to fade as you can see in this picture.
After all the time and effort I put into them, then the $25-30 in supplies I spent, it wasn't worth it for the results I got.
My F350's lights have been on for a year now and still look like they just came out of the paint shop. The clear he used is very durable too. A few times I thought I scratched them with my keys or something, but after examining them, they were still good as new.
If you want, box up your lights, send them to me and I will have the guy do them for you. Only thing you have to worry about is shipping on top of the cost of the job. Or you could probably send them straight to him.
He has done 3 cars for me; my truck, my fiance's Cadillac, and my Mom's Cadillac....all came out perfect, dark but with great light output.