+1 for everything the bear says above.
On the last point, though, a bit of speed adds stability and makes many things easier, so it's a tricky compromise.
A few other thoughts:
Traction: This will vary a lot over quite small spaces.
Look ahead, and plan where you are going to brake/steer. If you can straight-line mud or gravel, then brake/turn on the grippier bit beyond it, do so.
Muddy ground: On slippery surfaces, avoid cross-cambers.
Look ahead, and try to ride straight along the bottom of ruts, or completely out of them. As with tram lines, any crossing should be done at an angle.
Roots:
Look ahead; Cross at as near to 90 degress as possible. Shift your weight so that the wheel crossing the root is nearly weightless. (ie. aim to nearly wheely the front over, then lean all your weight onto the 'bars as the rear wheel comes over).
Rough ground: Try to avoid any big steps. This is obvious, but it's very easiy to follow the easiest line and find it ends in a big rock. By
looking ahead you can choose a slightly rougher line which avoids the more difficult bits.
Trust the bike: If you have confidence, it's amazing what you can ride over. If all else fails and you find yourself heading for a obsticle which you would rather avoid, stand on the pedals, relax (as much as it's possible to when outside your comfort zone!) and let the bike do its thing.
oh... And did I mention looking ahead?