The first thing to come to terms with is that you're never going to look as hard or big off of gear. 

Now that we have that out of the way, the number one factor that will help you keep your gains is always going to be diet. The smaller you are, the easier it'll be to hold gains. If a guy was naturally 145 lbs and under eating, then went on a cycle and gained 15 lbs of muscle, he is going to be able to hold this muscle pretty well. If he keep training and cycling, then in 4 years is 225 lbs, fairly lean, say 7%, maintaining his muscle is going to be a lot harder. 

For someone who falls in that later class, it may be impossible to maintain all of his muscle. At a certain point, it just isn't going to happen. This guy may start using peptides like IGF and gh, year round, and continue them, IF he comes off. Once off, he may lose some of his size, but it'll come back, once he goes back on. 

Another factor is going to be training and cardio. While off, you still want to train hard and continue pushing weight in the heavy range. If you dont give your body a reason to hold gains, it'll let them go. But you may want to back down the number of sets a little. You don't want to over tax, especially in PCT and while recovering. Same goes for cardio. Cardio will burn cals that could be used for muscle retention. That doesn't mean you need to cut out all cardio, but you don't want to push it as hard as while you are on gear.