My horse used to be needle shy and now doesn't mind shots a bit now. We put a chain over his nose so he couldn't pull away and went slowly. Let him circle around handler so he didn't feel trapped. Only one shot every few days at first (we gave most vaccs ourselves at that point), with a peppermint as reward.
I find that currying rapidly, intermittent with short "pinches" of your finger can cover up the feel of the needle.
I have a helper help me out with this. I'll have the helper between the horse's eye and me, so the horse can't see the needle as well. The helper will start rapidly currying and pinch-letting go, while I insert the needle. The trick here is to curry hard and try to disguise the feeling of the needle.
Be careful with twitches. I received a tooth breaking smack to the face after the vet twitched one of my mares to do a rectal exam/uterus ultrasound. The mare wasn't needle shy at all and fine with the needle and mild sedation but starting swinging her head violently after the vet put a twitch on her (said he always twitches despite chemical sedation).
As the handler at her head, I caught the wooden handle of the twitch hard across the mouth. Vet only said, "oops!" and handed me a towel to stem the blood flow. That resulted in two broken front teeth below and above the gum line and cost a small fortune to fix with multiple dentist visits for root canals and crowns, never mind the pain and swelling for weeks.
FWIW, the mare was calm as can be for the exam after the twitch was removed (at my insistence). Vet stood on the other side of the dutch door to do the exam and out of any possible line of fire.
Hoover arrived with a worrier's personality, a turbocharged reverse gear, and long practice at sitting back on stakeout lines (neckline or halter) for long periods of time if he wanted to avoid something.
He learned that Mano and I were Good Stuff after a couple months' living here. I can trim hooves, brush & tack up, most anything with him ground tied or with a lead through a blocker tie. Motorbikes can almost run him over and he won't bolt. But shots and known vet techs are still on the bogie list, and he reverts to his old ways with either one.
Update - Our vet was in the valley today so she stopped by with her deep bag of horse handling skills and almost as deep bag of needle size selections. Hoover tried his full gamut of tricks and histronics to avoid his shot. But, with 40 minutes of patience and skill with horses and syringes, plus a 25 gauge needle, she got his meds in him without threat to anyones lives or limbs. We think she's awesome - well, Mano and I, Hoover not so much.
Definitely, the best technique for Hoover is to have a very experienced person with a full set of good technique and good techniques do the shot-giving. Injection wise, he's a pro ride for the moment. I'll keep working on him, and on my horsehandling skills.