I always use a very thin putty knife (metal). You carefully place it between the head and the block and start tapping with a hammer. Eventually it will bury itself into the gasket and begin to separate the head from the block. If it's really stuck you might have to do this to more than one corner to slowly get it to come loose. I use the same method on intake when they are stuck. But it has to be a thin putty knife (scraper). FYI - Make double sure you have all the bolts out/loose, I've accidentally overlooked a bolt and kicked myself for it!!!
