Dos and Don'ts on the Trail - Basic Trail Etiquette

Here are some trail riding tips that I think most people know, but are always good to reiterate.

Do get off your horse every once in a while. It is good for you to stretch you legs and it provides a nice break for your horse.
Don't ever tie up with your reins.
Do balance your saddlebags to within a few ounces of each other.
Don't under- or over-cinch your flank strap (rear cinch). Cinched too tight and your horse may buck...too loose and it isn't doing its job. Worse still, he could get a foot caught in it kicking at a fly.

Do bring a longer lead rope...at least 10 feet long. This allows you to be able to wrap the lead around most trees twice, which prevents the lead from slipping down the trunk.
Don't ever tie to a snag (standing dead tree). If the horse panics, the snag can break and fall on him or someone else.
Do: If you must turn around on a narrow trail, always turn your horse to the downhill side. This allows him to be able to watch where he places each foot. If you turn him to the uphill side, he will invariably misplace a hind foot off the trail, a misstep from which he may not be able to recover.
Don't take your horse into water any deeper than his knees if you can't see the bottom. An old packer once told me that. Fording streams can be tricky. Large boulders on the bottom create poor footing and you should avoid them even in clear water. Avoid crossings that are immediately upstream from deeper water because horses tend to drift downstream. Once you have found a spot to cross, fix on a point on the opposite bank where you want to exit and steer for it.
Do be the first one to pull off when meeting other riders on the trail. It is dangerous to assume that the oncoming rider(s) will pull off in time or far enough.
Don't ever try to squeeze by other horses on a narrow trail. From a safety perspective, it is worth the extra time to turn around and ride back to a spot on the trail that is wide enough to pass each other safely.