When you reach the bonus stage the game will force you to play as a certain character. If you try the character and restart these levels the character will change and you will be locked out until you can afford a new skin.
Bubbles
Always make a point of popping the bubbles that occasionally appear floating around the screen as they sometimes contain coins.
Costumes
Try experimenting with the different costumes if you are struggling to complete a challenge as each outfit has it's own stat boosts.
Increase Scoring
To score extra points as you progress through the game just do tricks whenever you do a big jump and wheelies along flat ground.
Medals
Always try and complete the course objectives to get medals as you will need to have earned a certain number to unlock later stages.
Mud
Always try to use a ramp to clear mud entirely if you want to avoid slowing down in it. If there is no ramp available you can always perform a wheelie to slide through it with ease.
Bonus Stage Character...