I don't do cross-country but I had a back injury a few years ago and go this one from Apple: http://www.applesaddlery.com/riding/apparel/c233932/c378010648/p165..., which seems to be pretty good, even after about 4 or 5 years. I haven't fallen on my back since wearing it, though, so it hasn't been put to the test. But it's comfortable and I don't even notice it anymore (I wear it every time I ride).
Hope that helps!
Emily, your fellow Antigonowherian