There are countless posts on the best way to train biceps, the optimum split for getting huge, how to bench press properly, or any of a million other questions on how to become bigger, leaner, or break through plateaus.

But one technique that helps achieve all of these goals is very seldom discussed: De-Loading. A de-load is a planned reduction in volume or intensity (usually for one week, or one cycle of your training split), whose purpose is to allow the body to dissipate accumulated fatigue, allow you to fully recover, and prepare you for further gains. Also, remember that weight training does not just tax your muscles. It also puts stress on your joints, ligaments, connective tissues, and central nervous system.

Why should you De-Load:
To allow your joints, tendons, ligaments, and other supporting tissues to repair.
To allow your central nervous system (CNS) to recover
To give yourself a mental break from the intensity of heavy lifting
To reduce the risk of under-recovery (overtraining)
To prepare you for greater gains
Experienced lifters know that you can't go 100% all out in the gym all the time. Your body can't take it, and you can't keep up that mental intensity forever. If you try to, you often wind up getting injured, start just "going through the motions" in your workouts, stall out in your progression, and perhaps even give up completely.

If you de-load at regular intervals, you will find that over time you will make better progress, reduce your injuries, and keep yourself in the game mentally.

When to De-Load:

This depends on your experience & intensity level, your age & recovery ability, the program you are following, and many other factors. If you are new to lifting, you lack the ability to overtax your CNS, muscles, and connective tissues as much as a very experienced lifter, so you may only need to deload once every couple of months. If you are older and have a reduced ability to recover from weight training, then you may need to deload as often as every couple of weeks. In general, you need to set your frequency of deloading according to how hard you train and how quickly you recover. Somewhere in the range of every 4-8 weeks will work well for most people.

Signs that a de-load may be in order:
You feel tired, persistently fatigued, have a decreased desire to train, or other symptoms of under-recovery (overtraining).
Your weight progression is stalling and you can't seem to increase most lifts
You are experiencing aches, sprains, tendinitis, etc.
You train regularly
Note that last point again: If you train regularly, then you should de-load regularly as well. In fact, a regularly scheduled de-load should come before you start exhibiting any of these symptoms.

How to De-Load:

A de-load is a planned reduction in either volume or intensity, usually a week long (or one training cycle of your split). How you do it is up to you. The main thing is to back off your total effort to about 50-60% of what you would do during a normal training week. A few examples of how to train during a de-load week:
Do your normal routine and normal volume (sets & reps) but reduce the weight you use to about 50-60% of what you normally work out with for each exercise.
Use the same weight as you normally would, but drop your number of total volume (sets x reps) to 50-60% of your normal volume. (Note that you should stick to an 8+ rep scheme here.)
Train muscle groups that normally don't get a lot of attention
Use light weight and focus on refining your form and technique
Decrease your lifting and increase your cardio

... or any combination of the above. The main thing is to make sure that at the end of the workout you still have a decent amount of "gas in the tank". Personally, I prefer to de-load by dropping my weights to 50-60% of what I normally use, stick with the same volume, and focus on refining my form, technique, and mind-muscle connection.

If you want, you can even just take a week off entirely. If you know you are going to be on vacation, for example, just plan your training around it so that you can use that time as a de-load period. You'll be training smart and not feel the need to try to find some way to work out when the rest of your family is relaxing.

Summary: