The Faux Pas: This move is meant to target the butt, but it's easy to just "go through the motions" and let the hamstrings take over.
The Fix: Don't cheat! To make sure you engage the glutes, "plant only your heels on the ground, driving through them as you perform the movement,"
The Faux Pas: Too often, we sees people swinging a barbell or dumbbells up and down with momentum, rather than isolating the bicep to drive the weight up and lower it down. The dead giveaway you're doing it wrong? If your pelvis rocks back and forth as your forearms lift and lower.
The Fix: Tuck the pelvis slightly under, and keep the abs and glutes engaged with a slight bend in the knee and the head and chest up.
The Faux Pas: "When people attempt the overhead press, they generally wind up with a tremendous arch in the low back,". That arch is a dead giveaway that you're compensating with your spine to lift the weight above your head.
The Fix: "Tuck your tailbone underneath you, engaging the core and press straight overhead." Squeezing the glutes can also help. If you're still arching, back down to a lighter weight you can comfortably lift with correct form and work up.
The Faux Pas: "The mistake with lateral lunges is that people collapse at the knee instead of the hip," which will greatly tax the quad muscle without much help from the butt or hips.
The Fix: "Focus on bending at the waist by engaging the glutes, allowing you to keep the back straight,". Sit back into the lunge, almost as you would when doing a squat.
The Faux Pas: "Most people have trouble synchronizing the movement, because they're making it too hard on themselves," It's easy to collapse the elbows in, limiting the twist of the movement. Plus, like with normal crunches, you may be tempted to round the spine and pull on the neck.
The Fix: "Instead of putting the hands behind the head,you can perform this movement on a bench, and use the hands to hold the bench on the bottom. This will stabilize and allow a more complete range of motion." Remember to move slowly as you bring each knee into the chest for maximum abs benefit.
source : http://www.huffingtonpost.com