For random CDs, I have colored sharpies that I use to circle certain tracks. Good dubstep tracks get boxed in Green. My trance tracks are in Blue. High energy house/dance/top40 tracks get circled in Red. It makes it very easy to spot tracks in a certain style when you're looking at a cd wallet on the fly. If you couple it with my 12 track method above, it's great to find the right track at the right time.