Step 1: Better Pedals
Problem: The spikes that grip your shoes on mountain bike pedals are bent forward so they don't grip.
Solution: Use a vice grip or pliers to bend the spike forward.
Note: This will only work for pedals with metal spikes. Don't bend back and forth many times or the spikes may break. This worked fine for me but i am not responsible if you break your pedal(s) doing this.
Step 2: Make your gear shift easier to read!
Problem: When going fast, the handlebars move around too much to tell what gear you are in.
Solution: Hide all incorrect gear #s with tape.
Note: Don't use tape that leaves sticky residue if you think you will want to take it off sometime.
Step 3: More comfortable handlebars!
Problem: When going off jumps or mountain biking, grippy handlebars may cause sores on your hands if you don't have gloves.
Solution: Add something less sticky then rubber like paper to handlebars. See pictures...
Step 4: Other Hacks
1. My chain guard. Made from inner tubes. Inspired by this Instructable, but a strip of rubber is wrapped around my frame and secured with gorilla tape.
2. My iPod holder. Picture below and possible Instructable coming later.