Working out is not just a matter of performing the movement as fast as you can, finish and move on to the next exercise. In order to receive the most benefits from the machine you are working on, or movement you are performing, controlling the resistance throughout the movement is critical to achieving the maximum benefits of the exercise, plus to your safety as well.

Good workout facilities will monitor the work of the patrons, usually making sure they understand the proper form and technique, before allowing them to go solo. If you are new to working out or to your training facility and you have questions , don't be afraid to ask for instruction to assure you get the maximum benefits of each movement.
Improper posture and form can lead to varied aches, pains and sometimes injury. By performing the movement wrong you also may inadvertently take the target muscle out of play and actually be working more of the secondary muscle structure instead.
When performing any movement or exercise remember these tips:
1. Posture and form are critical to the movement you wish to perform. Your body is a highly developed mechanism that has many predetermined safety features for your good health. Good equipment has been engineered to take advantage of these natural safety features. as well as the bodies natural leverages for lifting. In exercises not using machines your posture and form become extremely critical to not create a cheating effect that will put your bodies natural balance out of sync.
2. Perform the movements with proper form will help in the prevention of possible injury such as pulls, strains and hyper-extenstions. By using the correct form we use our bodies natural injury avoidance mechanism to build strength and endurance in our structures which in turn will help to build an even greater net of support for our natural prevention system. Remember working out is meant to benefit your body and health, don't create an environment counter productive to the main goal.
3.One of the main reasons for form break is often time restraints. Time is only a measuring stick we should use while performing our workouts. The goal is to get the most exercise with proper form rather than the most exercise.
Exercising should never become an issue of getting faster and faster as we work. Remember you have 2 solutions to increase your productivity and gains while working out. You can increase your speed (as long as you are still able to control the weight and are not sacrificing form) or you can move to the next level of resistance which will decrease your repetitions but increase your work thus creating more benefit (once again if your form suffers by increasing your resistance then you are not ready to increase)
Proper form and technique allows you to hit the target muscle much more efficiently and gives you a much more intense workout and greater benefits. If you are sacrificing form for repetitions it may actually take you 2-3 times more repetitions to receive the same benefits as 1 repetition using proper posture and technique. Even worse you may wind up not even hitting the targeted muscle at all and instead build the secondary muscles which are not as critical to a complete workout.
4.Proper form not only saves you time, it also saves you money and stress. Think about it. Will you be happy if you sign a contract to workout and you go religiously but yet you never seem to be making any gains. Sure if you never worked out before and your just starting you will see some gains initially, just because you went from dormant to active. But these gains will soon plateau if you aren't using proper form, you are in effect throwing your hard earned money spent on your gym membership, right out the window.
5. Using proper form and technique will boost your confidence in your workouts and your understanding of what your doing. As your confidence grows in the gym your goals will be achieved faster and your overall confidence in yourself will increase. This confidence will carry over into other areas of your life being a positive force for you to control other aspects of your life.
The confidence and pride you feel from accomplishing your goals through proper form and technique are unmeasurable. We all enjoy that feeling of accomplishment and control over our own direction.
6. Finally the best benefit of proper form and technique. Your going to look better. Sure this is superficial but we all want to look our best. The health benefits are great but it's nice to here a compliment about our outward appearance also. Correct form gets us there faster.
What are some of the most common mistakes.
1. Sit properly in machines. Do not position your body in ways to cheat, so you can lift more weight or do more reps. When standing activate the muscle through the movement so there is no swing or jerk to the movement.
2. Try to use a pace that works the muscle through both ends of the workout at the same pace. Your negative and positive movements will both create work, and because you are setting up a rhythm for your workout you will avoid the jerking motions which put added undo pressure on the joints.
3. Listen to your body, if the machine or movement you are performing is causing you pressure in an area that is not the target area for that machine or movement, you are probably doing it wrong. Do not just do it anyway ask for assistance to make sure you avoid any possible injury.
4. If you feel nothing at all in the direct muscle that should be effected by the exercise you are doing, this may be a product of improper form. This is not always the case however it may just be a matter of the resistance level or as simple as some movements we will not feel much but we will eventually see the results.
So remember all the pointers above and you will work your way to success that much quicker. Remember all the benefits of proper posture, form and technique and you will be the beneficiary of wonderful results. Good Luck.