1.  Don't pick a prickly pear even when it looks like there's no thorns (there's super tiny sharp, practically invisible ones all over the pear!)
2.  Don't conserve your water.
3.  Don't daydream on the trail.
4.  Don't go off trail when you're lost.
5.  Don't get lost.
6.  Don't forget to stretch after your run.
7.  A bottle of wine and a pound of candy DO NOT make good recovery calories.
8.  Don't run off the cliff.
9.  Don't fall.
10.Don't play loud music in your earphones on curvy single-tracks.
11. If you find an actual bathroom on the trail, do not use the seat covers -- not unless you want to spend the next ten minutes peeling sweaty seat cover tissue off your bottom.
12. Don't text and run, unless you want to eat dirt. : )  But perhaps you are more talented than me. : )
13. Don't forget to pack toilet paper (no joke! : )  I have this in the Do's and Don't's sections.
14. Don't assume you have the right-away.  I suggest give the person travelling uphill the right-away, especially mountain bikers -- they are after all struggling more than the downhill traveller.
15. Don't look down, don't look to the top, instead simply look straight ahead.
16. Don't let negative self-talk enter your brain.  If it does, whack it out of park as if with a baseball bat.
17. Don't forget to smile!
18. Don't let the gnats drive you crazy.
19. Don't worry about being over prepared (I think an over-packed pack is okay; it helps with training.)
20. Don't count on stashed water.  It may not be there.
21. Don't stop looking for rattlers.  As soon as you stop looking, you're bound to see one.
22. Don't keep running when you're overheated.
23. Don't bushwhack (unless you have to or it looks really fun)