I started doing XC a counple years ago and came from hunters where I was jumping max 2'9". So I had a pretty steep learning curve and I managed to be fairly safe.
First, I only ride expereinced horses. This helped me safely acquire the skills and experience to ride XC confidently.
I sit taller XC than I do in the arena over jumps. That way if the ground on the landing side of the jump is lower than take off I don't go over the horse's head.
I also would say that having a strong leg is the best thing to keep you safe. I do lots of stiruppless work to help with this.
Body protectors help, but they won't protect against all injury.
Don't do anything you are not comfortable doing - live to ride another day. :)
catsandhorses is an American living in Ireland and riding mostly XC with "Bob". She is looking forward to buying a new horse when she returns to the U.S. in January'2012!