skullcrushers on stability ball.
this will work your triceps and your core.
start buy laying on a stability ball with the ball mainly under your upper back. make sure to hold your abs tight and keep your butt raised so your body is straight.
hold a dumbbell in each hand and put your arms straight up in the air.
bend your elbows and lower the dumbbells down to either side of your head~or your skull! hence the name ;) make sure your arms form a 90 degree angle.
repeat. i like to do 3 sets of 15 reps.