My adjacent arm and pec usually cramp when doing tricep injections. I normally hit the spot in front of the mirror with my pinning arm and shoulder slouched down for accessibility. After I'm in, I move backwards to a wall and push the adjacent arm against it so I don't have to tense the other side of my body while pinning. Does this make sense?
If I'm pinning the left tricep, I lean against a wall with the right side of my body and inject. You want to inject slowly because you won't want a lumpy looking injection. I feel that even though my shoulders are big and my gear is good, that shoulder injections leave a noticeable injection site. Triceps take oils very well and the bolus of oil is well hidden. Maybe my shoulders just have lots of scar tissue... I dunno. I like tricep injections but you can only do them sparingly.