It really depends on what state they are in. Is there surface contaminants, is it yellowing badly?
A simple first step is to polish with a headlight polish, it might not really get that shine up though. There is the option to wet sand which is what I did on my old E85 which brings up a much better finish than any polishing kit, you got to be brave though as it will look dull at first and you think youve messed it up but a polish after and it comes up like new.
I would definitely invest in a lacquer UV protection last stage, you can buy wipes/fluid online, make sure you apply at the end and leave to dry for 24 hours, this will help ensure the longevity of your work. If you dont the yellowing etc. will be all over the headlight in a matter of months.