If you want to get REALLY careful:
Use one needle to extract, replace it with a fresh one to actually poke yourself.
Wipe off the top of the vial with alcohol before extracting.
Inject as deeply as your can safely. Meaning for alomst ALL men, if you have 1" needles and are injecting in the glutes, push the pin ALL the way in. There is actually LESS pain and less chance of swelling if you get it far underneath the skin.
Be smart about injection sites and volumes. Meaning don't inject a TON of stuff like every other day into the same spot. Try to inject into the same spot only 1x a week.