Athletes who struggle with push ups often let their hips sag, hold their breath, or strain their neck (resulting in the "chicken peck" movement). To avoid these common mistakes, make sure you are keeping your body in a straight line, focusing on your breathing, and keeping your spine neutral. Performing this exercise with a partner or in front of a mirror can help you check your form and avoid the common flaws listed below.
IF your hips are sagging: Focus on engaging or tightening your glutes, legs, and abs. Focus on lowering your chest to the floor and keeping your body on that straight line.
IF you're holding your breath: Focus on breathing properly, this means inhaling on the way down and exhaling as you push back up.
IF you're straining your neck: Pick a spot on the floor slightly out in front of you rather than rounding your neck. Tighten your abs and focus on good form.
Performing a push up is challenging, but if done correctly engages many different muscles and can improve your athletic performance. Practice at home with a cone, foam block, or pillow and try to touch your chest down to one of these objects. As you become stronger, take the object away and try to touch your chest to the floor. You may also want to start in a modified position on your knees, this advocates proper form from the get-go and often times keeps you from flaring your arms out. (Work those triceps!)