This is my favorite exercise for toning the back of shoulders and triceps. I use light weights and perform a high number of reps (25+) for this exercise.
How to do it: Come into a lunge position, with your back heel on the ground. Lean over your front bent knee as your lift your arm straight up by your side, top of the weight facing the ceiling. Lift and lower the 2-3 pound weight about an inch (30 times each side).