A few years ago there was an article in the US magazine Flying about this - the general view was not to do it
But if you do.....
Make sure you have someone other than the pilot doing the work so the pilot FLYS THE PLANE AT ALL TIMES..
Small 'planes often generate a low pressure area in the cabin - hence the embarrassing blow back Using a long flexible plastic tube with a weight on the end and FIXED to the plane seemed to work - the end gets outside the pressure variations around the aircraft - but of course it may strike the tail or wrap itself around the rudder......
Someone suggest attaching the ashes to hydrogen balloon and using a BB gun to pop it when it reached a suitable height..............