8 pounds is too much weight for firming up, you want use a lighter weight, probably 1 -2 pounds and more repitions. There are also resistance bands that also work pretty well for that.
Best exercise I've found (that I need to start doing again...
Is raise your elbow so that it's pointing straight up right by your ear...then with can of soup in your hand raise and lower your forearm... Do this on each arm...