Hold any of the main controller buttons during a front or back flip. You will do an alternate flip and get more points/boost.
Race bike for freestyle
A purely freestlye bike isn't always best for every freestlye track. If you're having trouble winning a freestyle event with a freestyle bike, try your race bike instead. Sometimes being fast over jumps is more important than getting air.
Get 1st Place in Every Event
Finishing the World Tour requires you to get 1st place in every event in the 10th stage. Do this, and you'll unlock character-specific ATV shells for every character in the game.
Holding down a face button while doing a front or back flip will modify your flip and give you even more points/boost.
In Freestyle events, go for the Star powerup as early as possible. The sooner you can pull off a special trick, the better.
If you can't place in an event, move on and come back to it later. Winning other events further in the Tour will unlock new parts to beef up your bike. Then come back and ace those troublesome tracks.
Finish in first place in all events in the tenth stage to successfully complete World Tour mode. You will unlock alternate ATV shells for each character.