There is no secret really, thats why everyone wants to vulch and no one wants to deack.
.50's are a pain anyway, you have to hold em on the targeted ack for a couple seconds, so use cannon armed airplanes, it only takes one or two 20's to kill ANY ack.
Secondly, when deacking a field try and hit the acks on the outside 1st, without flying through the center of the field (this is where most of the ack is concintrated.)
Speed is a big plus, it seems the faster you go the less likly you are to be hit, and the less time you spend in range. Dont make a habit of turning 180's inside the range of ack, when you turn you give the ack a HUGE target that includes your wings and tail surfaces, so make turns outside of thier range, zig-zagging around around seems to work somtimes,(especialy against MANNED ack) but dont depend on it.
Hope I helped
