Switchbacks (in the uphill direction) - as taught to me at a BetterRide clinic:

1)Put your seat at climbing height (optimal power)
2)Get low low low (chest/chin to the handle bars) - this keeps your front end from wandering. You think you're low, you're not. LOWER!
3)Scoot up on the saddle (toward the nose) -- this keeps traction on the back wheel while your low upperbody keeps the front wheel tracking where you want it to go. It's not the most comfortable place in the world to be on a saddle, but it doesn't last long and makes a difference.
4)Start wide on the outside, aim to finish wide on the outside. Make the corner as straight as you can.
5)Look ahead. No, further ahead than that! Look at your exit from the switchback as you enter it, as soon as you can see it, and look down the trail beyond the exit as soon as you can see that.

To practice, find a slope with a tree on it. Place some cones or rocks to mark the outside of the trail, with the tree being the inside of the switchback. Start with the cones/rocks allowing for a nice easy wide apex. Keep moving them in and trying again until you have a nice, tight, steep switchback.