I'd advise against using a screwdriver. Screwdrivers are hardened steel which means that it's going to totally muller the soft alloy headset cups. I use a cut off seatpost stuffed through the headtube (spreads the load a bit and is also equally soft alloy) or a random alloy rod I picked up somewhere. Also just hitting it with a hammer and moving it round. Once it starts to move, you'll be able to see the cup move slightly at the edge you've hit, so then you hit the opposite side to level it up and continue until it comes flying out . Good luck!