Well here's a tip, provided you got the basics skills down. Do enough tricks early on to get your character in "super shiney mode," which boosts your speed and adds protection from falls. A few 360's should be sufficient. When you're shiney, run into your opponent to knock his/her ass out, giving you a considerable lead. The rest of the way, just maintain your lead and do a trick every now and then to maintain your speedy shiney mode.
This works really, really well with Akari and Ricky since they've got great handling, and it's practically the only reliable way I've found to win on the harder circuits.
And I'm announcing my engagement to Akari Hayami. Wish me luck.