A mountain bike is very much about stopping to roll around in the flowers. And it's really different than road riding technique wise. Lot's of body english, almost like skiing. The advice about look where you want to go is crucial, you need to pick the line you want to take through the terrain and stick to it.
When you're not spinning, keep your cranks level. The kind of crouch it creates makes you more mobile on the bike, a dirt bike takes more pulling and pushing around under you to to handle. That road thing of one foot up one down will jam you up. You get stuck there, and it's easier to fall over.
If you hit nasty stuff, mud , sand, whatever, keep spinning! Try to keep a good cadence and compensate by shifting down into easier gears. That's what the little chainwheel is for, not just climbing. The second the legs stop you get into trouble. Backpedal if you need to, but as long as the legs are moving you'll stay upright on stuff you wouldn't beleive possible.
There's more- I always have more to say . But go try this and see what you think.

Lizzy