The Faux Pas: "With pushups, most people screw up posture. Instead of a straight line, they wind up looking like a curve. The collapse usually happens at the lower back, and is caused by a lack of attention to core strength."
The Fix: "Start with your butt higher in the air, and hit the ground chest first. During the set, actively draw the belly button in toward the spine, which will activate the core muscles."
The Faux Pas: "The number one mistake here is that people look down at their feet, and, where the head goes, the body follows. By looking down at your feet, you force your body to 'round' unnaturally downward,", which can throw you off balance or place unwanted stress on your joints.
The Fix: "Make sure you look straight ahead, keep your shoulders back and spine straight," Frequently finding yourself glancing down to gauge where to place your forward foot? Rather than worrying about foot placement, lunge forward a distance that simply feels natural
The Faux Pas: The biggest problem is curving your spine, which puts strain on the neck and back. Look familiar? Poor crunches put the back into an eerily similar position as sitting hunched over in a desk chair!
The Fix: "Instead of trying to bring the shoulders to the knees, think about bringing them straight up towards the ceiling. This will engage the abs without compromising the spine." Lift toward the ceiling until your shoulder blades clear the ground, then slowly lower your torso back down.
The Faux Pas: "The single-arm dumbbell row is generally well-performed in terms of the movement itself. What people screw up is their posture." Rounding the back can make your spine and shoulder vulnerable to injury
The Fix: "Instead, make sure to keep a straight spine and brace the core,". Lift the weight up toward the hip until the upper arm is level with the back.
The Faux Pas: Planks are generally a safe and effective ab exercise, but bad form can hurt the shoulders and back. "Like the pushup, the mistake here is that people collapse at the lower back,"
The Fix: "We fix this by placing something like a broomstick on the client's back, coaching them that there should be three even points of contact: the top of the head, between the shoulder blades and between the buttocks,"
The Faux Pas: "Most of what's seen is people doing quarter-reps," meaning you're likely not squatting as deeply as you can and should be. Part of the problem is starting with your feet too close together, which causes the heels to slightly lift off the ground as you bend.
The Fix: Keep your balance by more solidly grounding your feet in a wider stance. You'll stay steady and strong as you squat deeper, which in turn allows you to lower your body all the way until you could be sitting in an invisible chair.
You may also find your knees "drift inward, compromising stability," If you find your knees caving in, position the legs so the knees and toes point slightly outward. This will engage the glutes and keep the knees from drifting in.
source : http://www.huffingtonpost.com