here are some bits of advice i give to my students:

1) be patient with yourself. remember you are doing this to be outside and get some exercise. no point in getting all frustrated if something intimidates you. the hill has been there for thousands of years and will still be there tomorrow if you want to try it again.
2) if all else fails: keep your weight back, momentum is your friend, don't think so much and smile!
3) don't forget to breathe and to loosen the death grip on your handlebars. I think we all tend to get a little excited on the trail and not realize we are holding our breath!
4) practice practice practice. i think a lot of people equate mountain biking to just plain biking. a lot of skill and fitness is required. hey some people are naturals. most of us (including me) need to work at it.

Happy riding!

Han