In order to get the most out of your weight training sessions, you need to learn proper form.  Improper form can lead to serious injuries, which will halt your training progress, or a workout that is simply just going through the motions.  If you do not have the correct form, you may not be working as hard as you should be.  If you cheat and do not complete full reps you are just wasting your time. 

The bench press is the number one piece of equipment that people use improper form on.    People often cheat by using their entire body to lift the weight, rather than just using their upper arms and chest.Proper bench press form does allow you to arch your back slightly, but it does not allow you to lift your rear end off of the bench. 

In order to get the best workout on the bench, and to tone your pectoral muscles, the back needs to be pushed inward and down just slightly while laying flat on the bench.  This allows your chest to come forward a bit and allows for a much more intense workout.  Your feet should be on the floor, your rear end on the bench, and your back should follow its natural arching pattern.  Cheating by pushing your back upward or off of the bench is not only wrong, it can also cause injury.

The squat is another weight lifting exercise that is often done improperly.  Many people add more weight than they can truly handle and do not squat down low enough to the point where their thighs are parallel to the floor.  Proper squatting form means your lower back should be arched a bit to avoid bending, your knees should be bent forward slightly but not hyperextending past your toes, and your hips and buttocks should be toward the back.

When performing dead lifts you need to make sure your form enables you to control your movements with your legs, and hips, and not lift the weight with your lower back.  Allowing your hamstrings to help control your lifts will increase your overall strength.
 
Always make sure you control the negative part of the movement when weight training.    Keeping the weights under control and taking a bit of time to make sure your form is correct will allow you to get the very most out of your workout and drastically reduce your chance of becoming injured.