From building running strength and increasing hip flexibility to sculpting a nice lower body, the lunge packs a lot into one move. Maintaining enough space between your feet to ensure stability, looking ahead rather than down, and keeping your upper body tall and lifted will help you get it right.
How to: Stand with your feet hip-width apart, your abs engaged, and your chin up. (While you're there, go ahead and give your shoulders a roll too.) Step forward with your right leg, and shift your weight so your heel hits the floor. Lower until your right thigh is parallel to the floor, your right calf is vertical, and your left knee is almost at the floor. Press into your right heel to push back up to standing, then alternate sides.
Variations: The lateral lunge strengthens and stretches the inner thighs, while switching sides with a jump rather than a step will get your heart rate up. If your knees wince at the thought of any of this, try stepping forward rather than backward with the reverse lunge until you can step forward with control.