Mountain Biking and I have a troubled past, I tried to get into it about 5 years ago on a borrowed bike who's chain kept coming off. We parted on bad terms after I fell off and cracked three ribs on about my 4th ride. I haven't spoken to mountain biking since.

However, my boyfriend has just convinced me to give it another shot. I'm a better road cyclist then I was 5 years ago and understand much more about bike balance, shifting, etc. For the time being he's lending me his bike since he just ordered himself a new mountain bike. We talked about it last night and think it will hopefully fit me as I'm 5'9" and he's 6'.

What do I need to know, what advice does everyone have? I'd like to be able to do this with him on occasion although I suspect that the road will continue to comprise most of my saddle time. I am looking forward to increasing my bike skills.