To tone up your upper body for strapless dress and tank top season, push-ups are my favorite. I love that you don't need any equipment to do them, and they're so effective that it only takes a few reps before you start feeling it. Here's a challenging variation that will also increase flexibility in your lower back.

To learn how to do this exercise, read more.

• Come into plank position (top of a push-up), with your hands under your shoulders, and your body in one straight line.
• As you bend your elbows out to the side and lower your torso toward the floor, step your right leg through the space between your left hand and foot, and cross it all the way over to the left side, resting on the outer edge of your foot. Keep your leg straight if you can.
• As you straighten your arms, step your right foot back so you're in plank position once again. Now lower your body and cross your left leg over to the right side. Straighten your arms and come back to plank position. This counts as one rep.
• Complete as many reps as you can. Don't be discouraged if you can't do too many, since this is a really challenging exercise.