Whenever you are performing an exercise ask yourself what is the goal of this exercise (G.O.T.E.)? The outcome of an exercise will only be as good as the precision with which the motion is preformed and the exact way it is challenged. It boils down to the quality of each individual repetition.
Exercise occurs on the inside. We must know what is going on inside as well as the invisible forces that are occurring with exercise. I learned this through the RTS course I am currently taking. It has changed the way I view exercise.
When choosing an exercise, it is important to know what muscles you are working and what you are trying to accomplish with that exercise. Knowing these are important when shaping your body.
See also: The Best Cardio Workout To Lose Fat