Here's a few things I keep in my field box:
1. Assorted screws and nuts. Get the black oxide kind, not the stainless steel.
The Ss screw heads are soft and easily stripped.
This is the kit I recommend,
http://www.tonysscrews.com/product.sc?productId=267&categoryId=61
You can create your own screw kit from your local
hardware store, or ebay sells kits for cheap.
2. Glow plugs
3. A back up of fully charged batteries, especially EZ Start wand batteries.
4. Zipties and body clips
5. Hex wrenches, small standard and Phillips head screw drivers,
glow plug wrench, assorted nut drivers, needle nose pliers,
6. I find a glowplug ignitor to be much less problematic than the Blue wire.
Its a good back up to have.
7. fuel tubing
8. temp gun
9. duct tape
10. compressed canned air
Things I most frequently damage or break:
Traxxas plastic bulk heads and control arms
Traxxas steering servos, hinge pins,
turnbuckles and tie rods..
These are all items you should consider upgrading if you're continually breaking them.