Bonus stages force you to play as a certain character. Just don't try them and restart these levels: the character will change, and you will be locked out until you can afford the new skin.
There are two ways to jump: tap the screen, or hold your thumb down (to duck) then let go to bounce up.
Try to do a trick whenever you do a big jump, and do wheelies along flat ground as it will increase your score.
You should always perform a wheelie whenever you're on a straight piece of track.
To avoid slowing down mud make use of a ramp to clear the obstacle entirely or perform a wheelie to slide with ease through the mucky ground.
Finish chalanges work for medals. To unlock later stages in Joe Danger Touch, you'll need to have earned a certain number of medals.
Pop those bubbles that you sometimes see floating around the screen. Some of these contain coins.
Costumes aren't just cosmetic costume changes because each outfit has its own stat boosts too, so if you're struggling with a challenge try experimenting with a new look for Joe.