They don't even need to be expensive. I have no clue why these aren't more common, but keep a cheap set of gunsmithing screwdriver bits around. Never seen them in a hardware store, but they are far superior to regular flathead bits with a chisel shape. Or just regrind all your screwdrivers when you buy them, which also makes it really easy to identify which flathead screwdrivers are reserved for hammering/prybar duty.