Get into a seat. Grab a pair of dumbbells and lean forward so that your back is almost straight, like your looking at your toes. Put your hands with the dumbels under the seat and then lift your arms straight up while keeping your back and arms straight. It looks like a wing flapping kind of motion. Lower the dumbbells to the starting position and repeat.
Another exercise is a type of cable cross. Put both pulleys at floor level. Grab the left side pulley with the right hand and the right side with the left hand, so they both are crossed in front of your body. Lift out and up with each arm straight like in the other exercise and do it as high as you can go with a good technique.