Lie on your back, with knees bent (like a sit-up position).

Take a 5-pound weight in each hand and do a fly:  a fly is arms almost straight -- over chest in air -- then go outward in semi- circle, one arm going to each side, to just above floor and return to over the chest in the air.
 
BUT the hand position is the important part: You have to have your fingers turned TOWARD your face, so the ends of the weights at the little finger end come together at the top.  This gets the “under arm dingle dangle”.  It’s the BEST exercise for that!!!