Complete all the tracks and win a gold on the San Francisco race to unlock Phat Earl, Miki, and Skier X.
Mode to Cheat
Go to the screen which says: "Press Start", hold L2 + L3 + R1 + Down + Square + Circle. Do this in this order or else it will not work properly. All skiers including Skier X, costumes, and ski units will be unlocked.