You should spend more time standing up on the pegs while riding your dirt bike, whether motocross or enduro. Sitting down is reserved for long straits, tight turns or motocross style cornering.
The best enduro riders today have a trails background and they tend to stand all the time (trials bikes have no seat by the way).
The elements of the basic or neutral standing position are:
That is the basic riding position. You can practice this position with the bike on a stand.
It is important to keep consciously correcting yourself while riding, for a while: repeat the elements above in order and check them. This is an important element of getting better fast.
In this standing position you have the most efficient control over the bike and can react quickly to compensate for balance.
The preferred position is having the balls of the feet on the pegs. This avoids injury when clipping a stump, avoids dragging the rear brake or downshifting by mistake and also affords the most control.
To change gears or use the rear brake, you have to move the foot so the arch rests on the peg. This is also useful when jumping and landing - it minimizes torque on the ankle when landing, avoiding sprains.
In the woods, when sliding into turns, you may need constant use of the rear brake - in this case, you're not moving the foot back and forth, so be careful of stumps and other dangers.
Gripping the bike well with the knees and ankles is an important element of control. You can see the difference if you ask a friend to shake the bike with you standing on it: while gripping it well versus while not. Gripping the bike well with the knees also relieves stress on the hands during braking and acceleration - you will appreciate this during long rides.
When cornering while sitting down, the outside leg should weigh the peg and also push into the tank, as a normal extension of gripping with the knees - this will keep the rear wheel on track and the bike planted through the turn.
You keep your back slightly arched to absorb impacts better and keep the upper body flexible.
Having a strait back or curved the other way is bad for your health. It also is less flexible when moving around.
The arms should not be locked (i.e. straight) but bent, to allow more flexibility. Keeping the elbows up is another element of control. You can push the bars better that way and also keep hold of them easier, when the front wheel deflects.
Again, you can ask a friend to shake the bars while holding them with the elbows up versus not.
While sitting, you have less control of the bike. It is harder and slower to move about, sideways or front/back for instance, to react to a loss of balance.
This is a resting position mostly. You should adjust the levers for the standing position and it will be a little awkward while sitting - for instance, if I sit down a lot, my knees start to hurt after a while from controlling the rear brake.
In corners, the rule of thumb is that you keep standing on the bike at less than 80 degrees or so. You can sit down for tighter turns. This will become more consistent with experience.
It is still important, while sitting, to grip the bike well with the knees and keep your elbows up.
Continue reading - Standing position part 2 for more details including videos etc.