Move yer bum. Simply - if you want more steering then move forward. Bent knees is the key. Unstable - side to side? Get used to the bike moving around under you. Even if you were sitting down the bike would still move, and you'd be moving with it too - throwing yer head around... Much better standing up - the bike can move around but you don't so are in better shape to control and see. Unstable - forwards / backwards? Yes - going downhill move yer bum backwards on the bike. This will move yer weight further back than if you were sitting down. Reverse for going uphill. For sand, speed is your friend. You also need to stand up on sand - keep the front wheel light and yer bum back. For mud it is all on being smooth with everything! Masterly? Not me, I just know what to do.. after I haven't done it. Siting and thinking before you try something is good. Good luck with the recovery.