You probably shouldn't ride a trail that terribly muddy if you can avoid it. If you must, for the trails sake, ride through the center. Don't try to skirt the edge of the mud where the ground is relatively still untouched. That just makes the muddy section wider. I know sometimes I'm hypocritical with this, but as often as possible it's best to ride right through the middle, keeping the area as compact as possible. The ground will recover sooner.