A microfiber cloth should be enough. You can get them at an electronics store, or eyeglasses place, and they also sometimes come with screen protectors. If needed, just breathe a bit on the screen, and the condensed moisture combined with the microfiber cloth should be enough to remove any sticky "goo" caused by the old screen protector. I didn't need to use any harsher means than that when I replaced my screen protector. I basically use the same process for occasional cleaning of the screen protector itself, after installed.
if your looking for a new screen protector. you might wanna check out boxwave. they have reusable ones that you can remove and reapply without leaving smuges on your screen. PLUS it comes with the microfiber cloth thingy.