so far I have:
-All tracks are from the same year (I did a "best of 2008" playlist, that worked out well).
-Trying to keep within a certain type of music (although this can be difficult, as some songs just don't fit along side hardly anything).
-A compilation of songs by the same band (only possible when dealing with bands, that have released a decent amount of material)
Any other ideas or tips? I keep wanting to create playlists, but I can never think of how to go about it. Maybe tell me what you guys have put together, that might help.