IM injections

Use a smaller needle.

I concur with this suggestion. Instead of a 1.5 inch needle, it is acceptable to use a 1 inch or a 1.25 inch needle for IM injections.

For individuals without much muscle mass, I prefer the vastus lateralis (thigh) or the ventrogluteal region (buttocks), although I realize these sites are not terribly feasible for the healthcare setting where the OP works.

I used to work in a clinic where we gave monthly injections of Synagis to preemies to prevent them from contracting RSV. We used 23g 1" needles. Chicken wings have more muscle than those little legs do so I developed a technique so I could stay off the bone when I was giving the shots. You can grasp the barrel of the syringe in the palm of your dominant hand as though you were going to "stab" someone. Position the syringe where about 1/2" of the needle is extended beyond the ulnar edge of your palm. You actually place your hand on the patient in a position where you want to give the injection, but rotated out slightly so the needle doesn't pierce the skin. Once your hand is in position and stable you can rotate your hand inward to inject the medicine and control the depth of the needle. You can withdraw and depress the plunger with your other hand, or if your hand is big enough, you can operated the plunger with the thumb of the hand you're holding the syringe with. I guess I'm calling this the OldDude technique. It takes a little practice but after I started using the OldDude technique I never hit the bone again.