So I've been more or less in off-season mode for the past year, my last prep was for the 2011 Tahoe Show last August. But for a lot of people, summer is contest mode every year, so as we start heading into another bulking cycle, let's go over a few thoughts for having the most successful off-season possible. How do we define success? Lean muscle growth with a lean starting point for the next prep. Credit to John Meadows, aka mountaindog1 on professionalmuscle.com, for formulating the four thoughts which were a basis for this post. I've re-worded to improve flow and inserted my own comments where appropriate, as well as noting which comments are purely his.
First and foremost: Do not get fat in the offseason!
At the higher levels of bodybuilding, adding 20 pounds to your contest weight is not necessary in order to make great structural and proportional improvements. Is it possible to add 20 pounds between shows for someone who is just starting out though? Maaaaybe, but it's doubtful that it would be quality poundage gains... A lot of bodybuilders get it in their heads that they need to build muscle as quickly as possible--adding fat to their frame "as necessary" in order to beef up as much as possible between shows. Unfortunately, this does not actually help muscular hypertrophy rates whatsoever. In fact, insulin resistance is very correlational with high levels of bodyfat. Building muscle gets harder the more insulin resistant you become. See my post on the negative effects of too much bodyfat, and how to combat the mechanisms which contribute to it, here.
Secondly: Don't rely on cardio for anything more than cardio.
Cardio can be helpful in the off-season for assisting in the routine, and healthful, maintenance of the cardiovascular system--but it's not a magical fat-burner! If it were, all those cardio bunnies who only do cardio and adduction/abduction would be ripped monsters, but they're not--diet, diet, diet! is the key to a tight physique. Oh, and spot reduction isn't real :)
Thirdly: The "Central Nervous System Boogieman" is only that, a boogieman.
Get the most out of the least with your training. Gradually build intensity and volume up. Your body will adapt to whatever you throw at it, so you can't just do the same thing over and over. Maximize each step in the process, just like you would do with a pre-contest diet. You wouldn't start your first week of prep with all your diet tricks out of the bag, would you? I hope not! When your body needs to scale back, it'll let you know. You won't be able to get a pump, you'll get irritable, etc. Don't not train really hard because of a fear you will overwork your CNS. That's garbage. Truthfully, everyone I have ever seen talk about fear of overworking their CNS, has not even looked like a bodybuilder. This one was so good I didn't reword it at all. I'll only add my own comment: Don't be such a pussy! If you're giving yourself enough time off between training sessions (for the same bodypart), and you're not overtraining within each session, you can go months, even years, without needing dedicated breaks longer than a few days.
Lastly: The off-season is not the time for drastically restructuring your training in order to "bring up" weaker bodyparts.
In order to maximally develop each muscle, many bodybuilders opt to back off strong point bodyparts in order to attach greater emphasis to weaker ones. In many cases, this occurs unintentionally and due to ignorance because bro only likes training arms and chest. And I've been harping on this in almost every post: The body is a system! We don't know exactly what it is that is going to create the maximal benefit for a lagging bodypart, but it could be that the bodypart in question will only be forced to grow through maximal training of even the strongest aspect of your physique. If your left arm is smaller, don't train your right less intensely! Do reconsider split organization if you feel something is being overtrained. For example, I recently decided to lump shoulders in with chest in order to train them directly and indirectly on the same day, resulting in more rest in between.
I found John's original post to be simply a thoughtful articulation of methods and principles my training partner and I were already using. I was especially happy to see the CNS issue dealt with--I mean really? Your central nervous system adapts just like any other aspect of your physique, it can handle explosive movement over and over and over--you might just need to grow a pair.