You really need to make sure that one nostril is covered and your mouth closed, and then just give a strong blow. Personally I try to aim more down than back.

Almost hit Thom's boss who was riding with us today. We were coming down off the mountain and I looked back and didn't see anyone. Then poof they were right behind me as I decided to clear my poor nose. His son is a triathlete so he knows the routine.

V.