1) shift into the middle (big) ring on descents.
2) is your chain the proper length?
3) also might look at the front derailleur. the limit screw might be set to allow the cage just a little too close to the seat tube, which would let it shift perfectly while allowing the chain to flop off.
4) use a jump-stop.
http://www.gvtc.com/~ngear/index.html
it's how i keep my ghetto 1X8 from jumping off the ring, so i know it will work for you.