One of the most often overlooked training principles is that of cycling. Also known as Periodization, cycling is perhaps your greatest tool to continued gains in the gym. It also works great on the diet, whether you're trying to gain mass or lose body fat. Either way, changing things up, in a strategic way, can really give you an edge in your bodybuilding and/or body shaping efforts.
There are varying degrees and flavors of cycling. There are microcycles, mesocycles, and macrocycles. I'll define these by way of examples.
Microcycles are characterized by their shortness of duration and their frequency. A microcycle might best be understood by thinking in terms of days whereas a macrocycle might be thought of in terms of months.
- Microcycle => Days
- Mesocycle => Weeks
- Macrocycle => Months
Your training split could be thought of as a microcycle. You don't train the same body parts, using the same exercises, sets, reps, and weights every day; you cycle your workouts. Generally speaking, a 4 day split routine really is nothing more than a microcycle of 2 workouts followed by a day (or two) of rest. Guys on a "Hardgainer" program might have a microcycle that consists of a 3-way split, cycled over the course of a week (legs on Monday, chest and arms on Wednesday, and back and shoulders on Friday, for example).
I personally cycle my nutritional supplementation, too. Every Thursday and Sunday, I lay off them completely, even if on a heavy phase.
A mesocycle may be one where your general workouts change over a 3 to 6 week period. I almost never have the same workout, but I keep the same split and generally keep the same or similar exercises (it's the weight, reps, and sets that vary); however, after about 6 weeks, my mind (and probably body) starts to get bored and I need a change (as you know, all progress – and failures – start at the brain). Such a change is actually positive on a few counts.
First, the aforementioned mind problem. Second, the body has little time to adapt to a workout if it's changing every 3-6 weeks. Finally, your muscles are better stimulated (think "all around") with a variety of exercises, sets, reps, and weights. Different muscle fibers may come into play with different workouts.
Finally, a macrocycle could be thought of as a pre-contest, contest, and post-contest training (if you're a competitive bodybuilder or VERY serious about your bodybuilding). Generally, bodybuilders train for many months, attempting to add mass; then they get into contest mode, where they're shoring up small weaknesses while stripping themselves of as much body fat as possible. Then, there's the post-competition training and dieting, where they might go on a break and then resume with their pre-contest program.
Personally, after every 3-6 weeks, I like to take off a week from weight training. I don't become a couch potato, of course. I don't want to see all my hard work vanish. What I do, however, is take a week off from the gym. I may go on several bike rides or hikes. I take along my hobbies, too. For example, I like photography. I will carry my trusty digital SLR with me on a hike up Half Dome.
Every year, I also take off a month from "regular" training. I might take up some "Caveman Training" or some body weight exercises. I may take up some very low volume powerlifting. Nothing that will kill my joints (the idea here is to rest the body and mind, let them heal from nagging injuries), but just experience something different.