To the up and coming surrogates:
Use a heat pack after intramuscular injections. It really does make a HUGE difference in soreness the following day. I did the injections without a heat pack for about a week and I can tell you it really makes a world of difference to use heat for 30min afterwards.
Have a bandaid ready. You don't want to be running to the medicine cabinet with blood dripping down your cheek. Sometimes it won't bleed at all, other times it's a sizeable dribble.