1.Pop wheelie often to help stay in first.(Bike of course)
2.Keep banana peels to block shells(all except blue.)
3.Drifting via Manual instead of automatic.
Manual gives boosts. Also a good tip is to purposely fall behind do you get good items, then pick the pace up a bit and use the overpowered powerup. Using a bullet bill in 4th is almost guaranteed to give you 1st place.