I had little success with mass market solutions such as skip doctor etc... found them to be junk and way too inexact...sometimes left cd worse.
Finally found a great solution...been using for years now...it's basically a motor with specialized fabric wheels and compounds...lists for $130 now...MORE than payed for itself...yes, there are some discs that could not be repaired (especially those with deep concentric scratches) but this has been a excellent product.... the supplies I got with it will last multiple lifetimes...so don't worry about that. Has also come in very handy when watching the occassional skipping movie...just pop it out and buff it...usually works fine after....
http://www.caswellplating.com/buffs/plasticbuffkit.htm