Classic NES Series: Excitebike Cheats for Gameboy Advance
Use a small amount of your turbo when you are going to go off a jump.
Press A repeatedly when your character has fallen off his bike and is getting back to it. He will shake as he runs and get to the bike faster.
You can go off the screen by making a custom track with the recovery thing and the ramps. Put 9 laps on and on the 9th lap your racer can go off screen.