Over-the-head extensions with a dumbbell. There's all kinds of different extension exercises. If you google, "tricep exercises" you can see various demonstrations. I recommend over-the-head extensions. They work great! Dips are good, and tricep presses too. Good luck!

Another exercise which works great is take a stick (broomstick works), put it behind your back, grasping the stick behind your back with your hands hanging down on either side of your body (palms facing backwards) and only apart as wide as your body. Then keeping your body straight, raise the stick up as high as you can behind you, keeping your arms straight. Count to 15, release and repeat. Has done wonders for me.

i agree. you need to do lots of light reps to tone the muscle. if you use heavier weights you are going to bulk up!