You might want to play with and practice the basic body position for mountain biking, called either the attack position or the neutral position. Pedals level ( 3/9) standing up - extending but not locked, but not just a little crouch off the saddle either. Eyes forward, no death grip on the handlebars, just one or two fingers on the brakes.

This is the basic body position from where all mountain biking maneuvers spring from. You ought to feel very balanced like this because you have 4 points of connection to the bike. When you sit on your butt, your are connected to the bike via an off balance triangle ( butt and hands) Think about it, what is more stable, four points of connection or an off balance triangle? practice this position - eventually it should feel very balanced and stable to you, and it gives you the advantage of being able to move your body around - especially back for descents. Scootching back on the saddle doesn't really count, you should let go of the saddle with your thighs, extend a bit and move your hips back.