I think its a good idea if we can all list the dont's we have learned with riding horses. My largest that sticks out is don't assume the horse has any idea what you are talking about if your not giving clear commands. Another is don't think that you are smarter than the horse. I have seen some stubborn ones that have no issues bucking you off if needed. :)

Don't expect your horse to not spook just because he hasn't before. Sometimes even the calmest horse will spook at something they've seen hundreds of times.

Don't forget that, just like kids, horses need plenty of play time and attention. And treats.

I would say don't yell at your horse. Also, don't jerk the reigns really hard to get the horse to go left or right. Horses mouths are sensitive enough to feel the pull of the bit in one direction or the other.