1 Staying in the saddle too much
The way that novice bikers keep their butts planted on downhills and rolling terrain makes me wonder if they are Velcroed to them. The saddle transmits every jolt and vibration straight to your buttocks and spine.
Use your mountain bike's primary suspension system.
I'm not talking about forks or seat posts, and I sure don't mean your butt. I'm talking arms and legs. Your wrists, elbows, shoulders, ankles, knees, and hips create a superbly flexible and efficient suspension system that will do wonders to float your torso and head above the action.
But the equipment only works if you use it right. Leave your rear on the saddle and you eliminate the suspension.
Instead, stand up. Keep your legs and arms bent and loose let them carry your weight. You can't ride this way all the time, of course, but do it when the terrain has you bouncing around.
2 Ignoring the front brake
Your real stopping power resides in your front brake. In fact, a skidding rear tire doesn't even slow you much, if it all.
While it looks spectacular with all its accompanying dust, it only tears up the trail and wears out the tire. The steeper the descent, the more you need your front brake. The rear one should be employed lightly, just to keep the back wheel from locking.
As an exercise, practice using just the front brake on descents. This can be risky, so start slow. Feather the front brake, using it with respect.
Clench too hard or stay too far forward and you might exit over the handlebar. As you gain confidence, gradually increase your dependence on the front brake. Then practice, practice, and practice some more.
3 Riding through water and sand
I know, charging into a stream is great fun. Water flies everywhere, and you get soaked. But so does your bike and its bearings.
This isn't so bad if you grease the bearings as soon as you get home. But be honest. Aren't you more likely to put the bike away in the garage until your next ride?
Meanwhile the water goes to work, and before long bearings and races are corroded. Your chain also becomes weaker due to the corrosion and will eventually decide to destruct in the midst of a particularly gnarly climb.
Sand is as bad as water and maybe even worse. Imagine all those fine steel balls smoothly rolling around in their coating of grease. Now add a little sand. If you doubt the possibility of sand penetrating well-greased bearings, ask the mechanics at many of the top cycling shops in Moab.
They don't have to reach far to find examples. Save yourself substantial grief and take my word on the horrors of this.
Carry your bike or ride slowly across streams, puddles, and sand. Otherwise, lubricate your bearings after every exposure, and oil your chain before, during, and after every ride.
4 Too much air pressure
Too many riders pump their tires until they're hard. Subsequently, they bounce more on every bump, increasing the roughness of the ride while decreasing control.
For traction and comfort you need fat, soft tires.
The fatter they are, the lower the air pressure required. Other factors that affect your perfect pressure include weight, terrain, and speed.
Don't worry about pounds per square inch; use your thumb to check the rubber's squish factor. Experiment until you discover what works best. You'll know your tires are too soft if you start experiencing pinch flats. Add a bit more air and they'll be just right.