If you want to be on the cover of any magazine, the best insider's tip I can give anyone is to shoot more vertical shots as you have to take into consideration the format of a cover. It increases your Pp (probability of publication) exponentially.
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Another tip is to call the editor and ask what they're interested in and send some shots. Hook, Approach, Wings of Gold and Naval Aviation News are always on the lookout for cover shots because they're much harder to find and need to be composed, lit and overall better images than editorial imagery.
Speaking as one of the guys who picks out what goes on covers.... ^^^what HeyJoe said...