Basically, stop doing the things you like and start doing the things you hate the most. It won't be fun. And it won't be what you're used to. And that is also why it'll make things happen! Keep in mind that the body adapts itself to the demands being put onto it. If you settle for a re-occurring amount of weight and intensity, why would the body bother to grow any further, once it has reached the point where it can comfortably handle that amount of work? Comfort is your enemy. Shock your body on a regular basis! When you've been training heavy for a while - take a break and go light with extreme reps for a week or two! When you've been doing all squats, do leg extensions! You get the idea.
This all sounds nice and dandy in theory, but how do you make this happen in reality? Fear not, oh ye brave but clue less. I'll give you an example of jumpstart-schedules to get you off the hook. See, I'm not only clever and witty, I'm helpful too. That's why they pay me the big bucks.
DOES THIS SOUND FAMILIAR?
Let's assume that you're an intermediate trainer, some experience in the gym, but not a hard core gym rat. For the past 6 months or so you've used pretty much the same weights, have hardly gained any muscle worth mentioning, and are currently training in 3-split of your body, allowing for a full week of rest between muscle groups. You train moderately heavy, in the 8-12 rep range for the most. You eat fairly well and have a protein drink or two every day. You should be growing like crazy, but for some reason, nothing happens.
HERE'S WHAT YOU DO!
Your problem spells: "P-L-A-T-E-A-U". You're not stimulating your muscles to growth, you're treading water to maintain what you have! Let's take on a 6-week jump start schedule to get you going again.
Here are the bulk of the changes you have to make. First off, look at your training log. Which are the re-occurring exercises week in and week out? Take out the most repetitive ones and put them aside for now. Which are the exercises that you seldom or never does? Is there any other reason than laziness why you seldom does these? (Discomfort in joints and similar problems disqualifies the exercise) The exercises that you know make sense for you to do, but never does, are in on the schedule for the next 6 weeks.
If you haven't made progress in the past 3 months, that's a sure fire sign that you need to jump start yourself! No progress means that your body has grown accustomed to what you're currently doing, and the only way to get off the hook is to do something completely different for a while!
Guess you're feeling kind of uncomfortable now, not doing your favorite exercises anymore. Well, it gets worse. Now I want you to change your weight and number of reps. If you previously did low-reps, say 6-8 reps on the most, it's time to do 12-15 reps. If that's what you're doing today, say hello to 6-8 reps for the next couple of weeks. You might want to phase in a little, but the basic idea is that you do it differently than before. If you were in the middle-ground, 8-12 reps, you can mix it up by doing a few weeks of low rep, and then switch to high rep for another couple of weeks immediately afterwards.
Weeks Three & Four
By now, you should experience soreness and strange pains in your muscles, similar to what you felt once upon a time when you started out. This is your muscles way of saying: "Hey, what the heck are you doing? We're not used to this! Look what you've done, now you're forcing us to adapt ourselves!" Of course, this is exactly what you want to accomplish. Adaptation equals growth. Pay attention to joint pain though, which is subtly different from muscle soreness. If you feel pain in the insertions and known "weak points" (shoulders, knee joints, back) it could be a sign that you should take a second look at the new exercises you're doing. You might cheat a little, without even knowing, but the result can still be extra stress on a vulnerable joint. If in doubt, ask a professional to check your form.
Apart from this, you should be right on track. Keep doing differently what you're already doing. You might get tempted to do some really crazy stuff, like extremely heavy 1-rep sets, but keep it sane. Injuring yourself won't help anybody.
Weeks Five & Six
Now your body should have adapted somewhat to your new training regime. That means, less soreness, better "groove", more comfort. There's no exact time stamp on this, so you'll have to pay attention to yourself - when the benefits drop, it's time to move on to the next step. The next step is - Surprise, surprise!! - to go back to what you did BEFORE week 1!
The logic behind this is the same as when you initiated your Jumpstart-cycle - when your body has grown accustomed to something, it's time to change it. Now, your body is accustomed to the new deal, so when you switch back to your old routine, you'll experience the same shock as during week 1-2.
So, you got yourself off the plateau. Congratulations! Now, all you have to do is keep this process going. Switch regularly, as progress seems to slow down. Also, keep in mind that it's always you, and only you, who can determine when it's time to switch strategy. Six weeks might be too short or too long for you - then just adjust it according to yourself.