This has been the best by far mojito I have had:
http://www.mywhimislaw.com/?p=1388
Pay special attention to the part about prosecco or asti spumanti. I prefer the Asti myself. It adds a whole new depth of flavor to it. And don't be afraid to go dirty with it. Dark rums can be just as tasty if not more so than clear/light/silver rums.