This is the most important element to any training program. Without it, you're ultimately doomed to fail in your physical endeavors. It doesn't matter if you're trying to build muscle, burn fat, boost endurance, or lose weight, without planning for this element your program is going to fail.
I know, it sounds pretty grim but it's a lesson I've learned the hard way and something I see every day at my gym.
Here it is:
If you don't plan to improve from workout to workout, week to week and month to month, it's only a matter of time before you stop seeing results.
Building muscle is a process. It's a slow and meticulous process but nonetheless, a process. You see, in order to build muscle, you have to break the lean tissue down through rigorous and intense exercise (See mistake #3). Through proper rest and nutrition, the muscle will rebuild itself and usually, the muscle will become stronger and larger than it was before. Essentially, the muscle has adapted to the exercise stress (stress point) from the previous work out.
This is an evolutionary response by the body where it becomes stronger, faster, and stronger in order to deal with the initial stress points (exercise).
Here's an example. Let's say you bench pressed 150 pounds for 8 repetitions in last week's workout. Also assume you didn't quite complete the last repetition (needed assistance to complete). This week, you managed to bench press the same weight for 10 unassisted repetitions An improvement of 2 repetitions.
What the heck happened? Essentially, your body has adapted to the weight and repetitions from the previous workout and has now become stronger. Not only has it adapted to the previous weight and repetitions but is now able to handle additional work loads. This is an evolutionary response where the body must adapt in order to survive.
Now, let me ask you question. What would happen if we keep everything the same, from workout to workout including the weight, sets and repetitions? If you guessed equilibrium you are right on the money. Equilibrium is that point in which the body is exactly in tune with its surroundings. Once the body reaches equilibrium, it stays there; in fact it wants to stay there. This is probably why it hurts so much when equilibrium is disturbed (vigorous exercise).
Since you've set a new standard for your muscles, they've adapted by becoming stronger. You gave your muscles a reason to grow. However, let's say for your next couple of workouts, you decide to keep your routine the same including weight, sets, and repetitions. What do you think is going to happen? Your muscles will stop getting bigger and stronger. They have reached a state of equilibrium and have no reason to adapt (grow).
In order to keep your body growing, it is necessary to avoid equilibrium. One of the more effective ways of doing this is to consistently try and improve with each passing workout. This may mean adding a few extra pounds to the bar, increasing the amount of repetitions performed, or speeding up the workout. This is something I like to call Beat your last session principle. Stated as follows:
1) Use heavier weight than the last workout for the same exercise for the same amount of repetitions and same rest periods;
2) Use the same weight as your last workout for the same exercise but for more repetitions using the same rest periods;
3) Use the same amount of weight and repetitions as your last workout but do the exercise is less time;
If you can manage to improve from workout to workout on a consistent basis, your body will have no choice to but grow. If you stop improving, your body will slowly reach equilibrium and stop growing. You don't want this so plan to improve. To do this, invest in a training log and track all of your workouts. Mark down everything to the last repetition and set. This will allow you the opportunity track your progress. For more information about this mistake, see this page here.