The pushup is a classic exercise that uses your body weight for resistance. If you have a tough time doing regular pushups, try the modified variety. Position yourself on your hands and knees with your eyes facing the floor. Place your hands slightly greater than shoulder-width apart and your knees comfortably apart. Slowly bend your elbows and lower your chest until your chin reaches the ground, then slowly return to the starting position. Keep your back straight and your abdominal muscles tight throughout the exercise.