One of the essential elements of training is progression. The principle of progression states that there is an optimum level of overload that must be accomplished in an optimum period of time. This principle is also known as progressive overload.
The idea is simple. As you train you should be able to slowly, but surely increase the load you are pushing. If you are bench pressing 155 lbs. for 10 reps today, the next time you bench press you should do 160 lbs. for 10 reps. It can also mean that you do the same weight, but more reps. Either way there is progress being made.
Progression is the principle that brings to light the need for rest and recovery. Constantly pushing the body harder and harder takes its toll. This can result in exhaustion and potential injury. With proper recovery and the right level of rest you reduce the potential for problems. At the same time, you avoid plateaus by allowing your lifts to continually progress.
I find that if you go too slow with progression you risk not making any gains and if you go too fast with progression you risk injury. Neither way is optimal. For me, small, 5 lbs. increases tend to work the best. I find that I am able to push the weight, avoid plateaus and avoid injuries. Those are key things if you are going to be consistent with your training.
One important factor in progression is that you must keep a training log. How can you possibly progress if you have no record of the weight you lifted in your previous session? Be sure to write down each set separately. Make sure that you log down your weight and the number of repetitions on each set. I also put an arrow next to the exercise to indicate that I need to raise the weights on my next workout.
Utilizing the principle of progression along with the other principles we've discussed will push your results to the extreme.