a good piece of advice is "watch the gap and not the obstacle", meaning if your trying to avoid a rock/log/sheep then concentrate on the route AROUND it, rather than keeping an eye on it all the time. otherwise, for some bizarre reason, it just seems to draw you towards it (?). i always remember this and i've been MTBing for about 16 years now, it's served me well. it's the only 'rule' i stick to and share with anyone, bar that, go as fast or as slow or as reckless as you like. it applies at pretty much all speeds, being rather more necessary the faster you go (it can hurt otherwise)
tyre pressures need to be lower for improved grip (but beware the snake bite punctures when you clout rocks with your rear wheel!). when going uphill off-road it's better to keep your bum in the saddle so your weights over the rear wheel. cleats are good for quickly hopping over stuff (but not essential).
o2gf74 is right with starting easy and then progressing.
i disagree with paulB, nothing is better than riding a filthy route in autmn/winter, getting proper clarted with mud and vegetation and sweat, coming home, hot shower, nice big cup of tea and cake on the sofa. bliss. my girlfriend thinks i'm an idiot
bing