Things I've found that make it worse.
Injecting cold oil.
Injecting into a cold muscle.
Injecting warm oil into a cold muscle.
Injecting cold oil into a warm muscle.
Injecting into a tensed muscle (this can actually be hard to avoid when you're doing it post workout to warm the muscle, and you have pumps on cycle. Let it rest for 5 minutes or so).
Injecting with a high heart rate - which also makes it hard to hold steady. Again, rest a bit post workout.
Injecting before letting the alcohol-wiped area dry.
Shaking the needle excessively.
Injecting way too fast.
Alcohol wiping straight after pinning.
But by far and away what makes it the absolute worst even if you do everything else above right - using 'dirty' gear. That is, gear that is laced with way too much Benzyl Alcohol to make it sterile, and/or using an inferior carrier oil - perhaps even just one that your body doesn't agree with.
The last stuff I used was utter garbage and the amount of alcohol content just irritated the site so much that I'd get Test flu, lumps and pains for days after from my body thinking it was being attacked. And I had to religiously workout first to warm up, warm the oil, pin, and then rush to the shower to use a hot water massage for 20 mins just to lessen the pain the next day. And it was always there, just in varying degrees. Thigh pins were out of the question because I'd be limping for a solid week. It would hurt when I was actually pinning, and even when it didn't, it always hurt for days after.
However.
I'm on some new stuff now for the past 3 weeks and so far, it's absolutely FLAWLESS. It's a mix of MCT oils (like coconut and apricot, and is paler and less yellow) not LCT oils and actually has just the right amount of BA. And I cannot ever feel a thing, unless I disregard all my points above, and even then I'll only get the dullest feeling if I really hit the area the next day. No flu ever. In-fact I'm even thinking of reattempting thigh pins now to give my delts a break.
My routine out of habit is still:
Workout, or at least do some stretches and movements to warm the muscle. Let it rest for 5 minutes.
Preheat the oven for 2 minutes, draw my gear, turn the oven off, open it up and hold the syringe near the doorway to let the hot air warm the gear as I roll it in my hand. (I don't like the idea of potentially contaminating the whole vial and using water etc, so I only ever heat what's in the syringe). Change loader needle (21) to pin needle (25) so it's not scorching hot I do this now, after the heating. Grab a tear of tissue.
Sit on a chair (feet always planted so you don't bounce on pointed toes and shake), pull one arm across my chest with the other (which seems to also relax the opposing arm, leaving the other to pin).
Slip it in, aspirate, inject slowly, pause, slip out and then grab my small tissue tear-off and hold it on the area to stop any bleeding and oil. Apply pressure with a little rub.
And then out of habit now I usually just go take my shower, wash my hands, then give the area a little rub.
Good as gold. ZERO pain. The quality of gear and what suits your body makes the most difference IMO.