Reverse the phase of any microphones underneath the drums. For instance, if there is a microphone on both the top and bottom of the snare, they will most likely be out of phase with each other. Reversing the phase of the microphone below the snare will put it in phase with the top microphone. This will deliver a full and natural sound, and can be done by a phase reverse switch on your mixing board, microphone preamp or plug-in.
Try reversing the phase of one of the overhead microphones on the drum set to make sure it is in phase with the other one. If the sound improves and becomes more full, then these microphones were out of phase. If the sound becomes thinner, the microphones were in phase to begin with.
Make sure each individual microphone is in phase with the overhead microphones. Flip the phase of each microphone and listen to see if the sound, most notably the low end, improves. Pay special attention to the room microphones. Depending on where the microphone is placed in the room, it could be out of phase with the other microphones. If the signal sounds more full when the phase is reversed, the microphone was out of phase. If the signal sounds less full than before, the microphone was in phase originally.
Move a microphone farther away from other microphones on the drum set if it sounds out of phase but its sound is not improved by reversing its phase.