I've been mountain biking on and off for the past year or so. Even though it's recreational, I still want to get better. I particularly have trouble with shifting gears, climbing and cornering.

When going up a hill, I always underestimate the gear I'm in a end up switching in the middle of a climb. When on a flatter surface, I find it hard to accelerate and catch up with my friends. Should I be shifting up front in addition to the back? When climbing, do I get out of my saddle or stay in?

My biggest problem is cornering. I crashed a couple times this week because I underestimated the turns. Should I be using my front brakes and how can I turn a corner smoothly? I'm fairly big (6 feet, 210 lbs) so this may also be a factor