Quitters never win, and winners never quit. Heard that one before? Well, it holds true all the way - if you're not sticking to the schedule, you'll never get as good results as if you would, period. This is not to say that a rest-break when you're on the border of overtraining is bad, but the 2-week spurts followed by 4 weeks of rest you often see are out of the question. If you're actually unable to follow the schedule you've set up - not by laziness, but because of reality - save yourself some trouble and change your schedule.
"I can't" is a phrase you should ban from your vocabulary. If your mind can't imagine success, your body sure won't do the job for you. Only when you can picture yourself actually pulling something off and come out a winner - may it be losing 5 lbs of fat or winning a contest - will it be possible. Did you see "The Matrix"? Even though the movie is completely fictional, it puts some light on the incredible power of the mind that we actually possess. Arnold proved what focus can do to a body as well as a career.
If you want to gain weight, a moderate over-consumption per day is the ticket, without going to extremes. On the flip side of the coin, if you want to lose weight, maintain a slight deficit over a long period of time. As always, the key is moderation and sanity. Discard the crazy ideas you get from the pro's. Stick with what is healthy and feels good for you, and the results will come over time. Nothing beats good ol' food.
Protein drinks are great, since they give you a boost of nutrition without triggering an insulin-release. Creatine monohydrate is another supplement that most weight lifters will find useful. A multivitamin/mineral capsule should be the mainstay of every active person's breakfast. Thermogenic enhancers, mainly the caffeine/ephedrine-combination, can do wonders in getting rid of bodyfat during a diet (given that you are physically fit and Ok'd by your physician, of course). Maintain certain skepticism to the latest "miracle supplement", whatever it is. If it sticks around and keeps growing, like Creatine, you've probably got the real thing. Something that was hyped like crazy for two months and then sortof faded, is much less likely to be a hit.
Kill Your Darlings
Bodybuilders generally love training chest and arms. Hamstrings, calves and neck seldom get the same enthusiasm. Guess what? Over time, it starts to show! There's nothing more ridiculous than a huge guy with spider legs, and your first step in avoiding this is to balance your training. A simple rule of thumb: Stop doing what you like the most, do the thing you like the LEAST first, and finish with the fun stuff. It's not fun, I know, but over time you'll appreciate it.
Don't Neglect Cardiovascular Training
You need to stay in cardiovascular shape not only to stay healthy, but also to be able to give your max when you work out with weights. If your lungs give up before your legs when doing squats, well, I think you can figure out what that'll do to your overall progress.
Listen To Your Body
If your body says it's overtrained, give it an extra day of rest. If you've got a lingering fever, back off the training, even though you might feel almost Ok. A strange pull or joint pain could be a sign of a more serious injury in progress. Stop doing that exercise immediately, and carefully try another way to train that muscle, that doesn't hurt. There is absolutely no point in pushing yourself through two more sets only becuse it's written down in the log.
Keep Track Of Yourself
Write down what you eat and what you train every day, preferably on an ongoing basis so that you get as accurate information as possible. This will later give you a hint of what you did right or wrong, when you can see the results a couple of months later. If you gained muscle, but also got pudgy, you will have an idea of the "calorie bandits" you should leave out for next time. If you've grown stronger, but suddenly hit a plateau, you can look back and see where your training got stagnant. Without a log, you're fumbling in the dark.
Remain Open To New Ideas
A lot of people let pride get in their way of results. They'd rather do bicep curls with 140 lbs using terrible form and get no results, than use 80 lbs with strict form and get improvements by the month. Don't let the joke be on you though. There's a lot of bad advice along with the good out there, so be your own judge of what makes sense to you. Try and discard things, and keep what works for you.
Understand The Big Picture
You'll live for many years. There is no "quick fixes" that'll bring you through the whole race. Your health should always come first, so be skeptical to any "magic" ways to lose fat, gain muscle, or improve stamina. Only a fool would start a marathon by a crazy, all-out 400-yard dash.