On a 76? I am guessing you have the small mirrors. If so, there's no way of tightening them that I know of. I have the same problem, unless you mean the mirror is loose on the mount instead of the ball pivot. Then you just have to take the mirro off with the phillips head screw, and bend the tab, i think forward, hook the tab in the mirror and tighten the screw tight. If it's the ball pivot loose then I think it's time to get new ones. On my 76 I gave up, don't wanna spend the extra $35 for mirrors from LMC.
If they are the bigger mirrors with a tripod mount, I am unsure, I have never really heard of them getting loose. Someone may chime in with some info.