One of my friends does this all the time and she got me into it...Take a regular song..Haunted by Evanescence for example, and play it with the opening or ending of an anime series. (Haunted and the opening to Sailor Moon, the eppys with Saturn, works really good!) Play the music to the anime low (or mute) and play a song with it and see if it matches! It's pretty fun and weird...we've come up with a few cool ones.
Try it yourself! If you find any cool matches, post 'em! ^^ And have fun!