The only things I've had to replace thus far other than the 4-in-1 and motors are blades and skids. I would heartily recommend the skids from boomtownhobbies.com; they'll take much more abuse than the stock skids.

The gears are not something that break often. Something you might want to do to prevent damage to the upper rotor head is to remove the screw that goes through it. Your flybar will probably pop out during a moderate crash, but it's easy to pop back in and the alternative is a broken rotor head.