I used to be a nurse and so I have given a lot of injections in my time. I remember that many patients were scared of intramuscular jabs because they can be painful.
We were taught a technique in nursing school that helped find the best place, when administrating a bolus injection to the buttock area. But like most things, some of us were better at it than others.
In case you're curious, the method involves drawing two lines on the body with your finger and aiming for the cross point. Often this alone was enough to distract the patient so they didn't even realise you'd given them the needle.