At the start of the race, lean forward and do a front wheelie. Hold it as long and often as possible. Get your boost rating up. Also, learn where the bumps are in the level and try to bunny hop over them. Make sure you buy speed upgrades to help.
Tricks will fill your trick meter. When full, it will initiate a boost. Keep filling your trick meter while boosting to multiply the boost. This easiest with Sir Colin or Farty Pants. They will spin so easily that you can do front flips by simply bunny hopping and rotating forward. Keep executing the tricks and the boost will blast to the x5 multiplier.