The pushup is one of the most perfect exercises. Toning and strengthening almost every part of the body -- especially the upper body and core -- pushups can be done anywhere and at any time. It's also easy to modify the pushup to make it easier or more challenging, depending on your fitness level. As with any exercise, it's important to do pushups correctly in order to avoid injury.
Begin on your hands and knees with your hands underneath your shoulders but slightly wider than your shoulders.
Come onto the balls of your feet and the heels of your hands, and then walk the feet back until you're in the plank position. Keep your hips lifted to avoid the lower back bowing so the belly sags towards the ground.
Begin to bend your elbows, lowering your body in one solid piece down towards the floor. Your elbows will bend out to the side, not behind you. Keep your abdominal and leg muscles engaged throughout the entire movement. Your head should stay in line with your spine; not droop.
Lower yourself down until your chest is about an inch or two from the ground and then slowly push yourself back up to the starting position. Push through the heels of your hands in order to return to the starting position.
Don't let your back sag down or your hips rise up. To modify, start with your knees on the floor and your hips tucked.
Start conservatively to avoid shoulder injury. Also, pushups should be avoided by anyone suffering from neck or shoulder injuries.