I've done this more times than I can count. Everything they all mentioned above is correct.
I especially agree with using an open ended wrench under the bottom of the injector, against the manifold boss, to pry and remove easily. Just remember, you're working with semi-delicate items, and finesse is your friend, as opposed to using "brawn" ie excessive force. If it's stuck hard, then something else must still be fastened somewhere. Re-examine before forcing!
Also, always prelube the o-rings. I prefer a silicone type, but good ol' fashioned vasoline works just fine too. (small evenly distributed amounts). This will help ensure not tearing or nicking the new o-rings. All new injectors I've bought came with new o-rings. If for some reason they don't, or you need more/replacements, you can purchase a set or idividually from O'Reilly and other parts stores. They're made by Borg-Warner, but I don't have the part number currently with me. 2 o-rings per injector ---> one in the fuel rail and one in the manifold injector boss. Hope this helps...