Technique is and always will be the most important component of exercise. All too often people set themselves up for failure by not paying attention to minor details of their workout. Progress is often lost in the gym by making small mistakes in and out of the gym that may not seem like a big deal at the time. Over many repetitions these small things will add up and form a habit that will become hard to break.
During my time as a trainer, I have seen many people who lack the attention to detail when working out. Many clients would have negatively affected their workout had I not been there to correct their form. When I say this I am referring to a simple concept such as, "make sure the bar is centered on your back" or "grab the bar evenly", not a complex biomechanical error such as foot pronation when squatting or shoulder internal rotation with pressing movements.
These types of cues, in my opinion, are very different as one is skill related to the individuals patterning while the other is simply carelessness over details that help optimal performance. These simple, immediately correctable errors in judgement can determine the level of success or failure of any particular exercise. In the example above, a potential back injury could occur from uneven bar placement. This same error often occurs when pushing/pressing a barbell causing an uneven load on the arms and potential compensation. No matter how much effort is used perform the exercise correctly (or look correctly), an individual could never fully achieve symmetry all because they failed to set up correctly.
Setting yourself up for success can also be applied to the design of your exercise program, aka choosing the appropriate exercises, reps, and rest based upon your goals and abilities.
Example: Bench Press
If your goal is to increase your strength or weight lifted in the bench press, you should be placing your feet firmly on the ground wide apart or on your toes under hips and below the bench. If your goal is to train the muscles in your chest, shoulders, and triceps (even though I think there are better alternatives than the traditional bench press), you should have your feet placed up on the end of the bench preventing yourself from using a good hip drive or the rest of your body for assistance. Neither way is "wrong" but one technique is a much better option for you to reach your goals.
Taking the time to evaluate what your goals are and determining which exercises and variations are better suited to meet those goals will determine how fast (or slow) you achieve them. Taking the time to evaluate what your current capabilities are is extremely important for avoiding injuries. For example, understanding your current mobility and stability should impact squat variations (front squat, back squat (high bar vs. low bar), overhead squat etc.) chosen for your program. An unknown restriction could lead to potential pain, compensatory patterns, and eventually injury.
Next time you're in the gym be sure to take your time when setting up your exercises and hopefully you (or your trainer) have done your homework in determining which exercises/variations are appropriate for you and your current goals.