The mechanism responsible for numbing your bum, is simillar to that responsible for causing pressure sores in the immobile patient.
As you rightly said, the symptoms are caused by diminished circulation to the area, caused by prolonged pressure of the body, (usually underlying bony areas), onto the skin and associated structures.
Unrestrained, well persons, sub-consciously move their positions slightly, every twenty minutes or so, to relieve the pressure.
I'm not sure about the praticalities of your cockpit, but avoiding action could involve remembering to shift your position every 20 minutes or so. (We have to actively do this for the immobile patient).
I'm a little out of the loop these days, most of my patients walk in and walk out (hopefully
), but there are pressure relieving devices available to help with the problem, the simplest being a sheepskin rug.
Happy flying