Hints:
Winning every freestyle competition:
In the freestyle levels, all of your opponents will ride like a normal
race. This is freestyle, so only trick points count. Find the biggest
set of ramps in the stage and hit them. Turn around and hit them again.
Repeat this for a lot of points. You will easily win every freestyle competition.

Volcano: Shortcut:
There is a shortcut in the Volcano level. Go to the left and keep going until you see trees. Go into the first set of trees and you will win the
race.

Winning race levels:
In the Race levels, the only real competitor that can hang with you is McGrath or Pastrana. There are two to four shortcuts in each level. Practice to find all of these in each level before you race. When you race, all of your opponents will ride the normal route on the first lap, then only McGrath or Pastrana will take the shortcuts during the second and third laps. By the time that happens, you would have already finished.