In order to work out the underside of your arms -- also know as your pits -- you have to hit all of the muscles that attach at or near your shoulder joint. That includes your pectorals in your chest, your upper arms and the muscles of your upper back. By exercising these areas, you will tighten and tone your armpits and avoid that bit of padding that so often peeks out around strapless dresses and tanks. For best results, perform theses exercises three times a week.
Lie face-down on your mat to perform pushups. Press your hands into the mat at shoulder level and push up, straightening your arms and lifting your body off the mat. Keep your body straight from your shoulders to your feet. Lower until your arms are at a 90 degree angle. Repeat for three to four sets of 20 repetitions. Pushups target your chest, arms and back.
Stand with feet shoulder-width apart to perform tricep kickbacks. Hold a medium dumbbell in each hand at sides with your elbows bent at a 90-degree angle. Bend your knees slightly and bend forward at the waist until you're looking at the floor. Straighten your arms and lift them behind you. Slowly bend your elbows. Repeat for three sets of 15 to 20 repetitions.
Bend your knees and stick your bottom out slightly to prepare for back flies. Plant your feet slightly wider than shoulders. Hold a dumbbell in each hand and leg your arms hang straight in front of you. Take a breath, exhale and lift your arms up and out until you feel a squeeze in your shoulder blades. Slowly lower your weights and repeat for three sets of 15 to 20 reps. This exercise works your arms and upper back.
Stand up straight and hold a dumbbell in each hand, palms facing inward, to perform hammer curls. Bend your right arm and lift your weight up to your ear. As you lower your right arm, bend your left arm to lift your other weight. Continue alternating arms as quickly as you can -- without sacrificing your form -- for one minute. Work up to three sets of one minute each. Try to move just your arms during this exercise, and control your movement so that you don't "sling" the weights into the air. You can also perform curls sitting down -- just be sure to keep your back straight.
Sit on your stability ball with a weight in each hand for pectoral flyes. Slowly walk out with your feet until the ball is under your shoulders and you're facing the ceiling with your feet firmly on the floor. Hold your arms out at either side, elbows straight. Slowly lift your arms until your weights are together over your chest. Lower to start and continue for three sets of 15 to 20. This exercise hits your pectorals.
Sit on an exercise bench or in a sturdy chair to begin tricep dips. Grip the edge of the bench with your hands and walk out with your feet until your bottom isn't resting on the seat. With your elbows pointing behind you, slowly lower your body until your arms are at a 90-degree angle and your upper arms are parallel with the floor. Raise yourself back up and repeat for three sets of 10 to 15.
Do 150 minutes of moderate cardio activity every week -- including swimming, dancing, rock climbing or elliptical exercise -- to burn excess fat as you tone your upper body. Running and biking are also useful for cardiovascular health and calorie burning, although the primary work of these exercises falls to the lower body.