The best thing to do is stay aware of your situation.
Other than that, most BG's aren't shootists and will probably miss more often than hit. I'm old enough to feign a heart attack which would likely save me. I've also had a real one so I know the procedure. I would also draw on the way down just in case.
I've thought the scenario through many times. Clutch the chest, turn your back and draw as you go down.................the rest are all immediate decisions given circumstances.
Young guys can have heart attacks too. Just have to be a good actor.