I pretty much use the same method as Doober's dad passed on to him, except I go over the arm, not under.

For a right-shot:

1) Left hand pinches left nostril
2) Lean right over the right arm, tilting top of head head back and to the left to ensure good clearance over the arm/shoulder
3) Blow like hell.
4) Reverse everything to clear the left.

The thing to remember is that your nostrils point down farther than you think so the lean/tilt is important.