Don't throw the weights up with your body, it's all deltoids. For this one, remember to not use momentum as well. Lift slower on the upswing, and resist the gravity on the downswing. Keep your back flat and abs tight at all times. This combination can be done seated or standing, whichever is more comfortable.
Most people prefer to sit.
Using dumbbells, holding them down at your side, roll your shoulders forward and up towards your ears for half of the reps, and backwards and up for the other half. Try to pause for a split second at the top for each contraction. Remember to keep your abs tight, knees slightly bent, legs apart to protect your lower back. Wear a back support belt if you can. Start with regular crunches, then immediately go into reverse crunches.