Good form will not only help to prevent injury, but will also increase the effectiveness of your workout.
While down time and sleeping may feel like lost time, if you're trying to build muscle, it's anything but. The body uses this time to recover, and ultimately to build.
Love weight training but not so fond of having to do cardio? Try doing just a couple quick minutes of intense cardio, like skipping or sprinting, between sets. Trust me it'll be over before you know it and you'll feel better because of it.
Whether you are trying to lose weight or gain it, trying to eat healthier or exercise more, tracking is extremely beneficial. Keep track of how much you've lost/gained/eaten/exercised, whatever it is that motivates you. This will also give you an idea of the habits and exercises that help you to or keep you from achieving your goals.
When it comes to weightlifting and bodybuilding, keep in mind that the quality of the workout is much more important than the quantity of the workout. Be sure to concentrate on form and intensity, rather than how long you're at the gym.
When lifting weights remember that lowering the weights in a slow and controlled manner is just as important as the upward motion.
Free weights help strengthen not only your main larger muscles, but also your smaller stabilizer muscles since they require you to balance and stabilize as you lift. In this way they can be more effective than machines.
Improvements are a great way to stay motivated. Since increase in muscle can offset decrease in fat, weight isn't necessarily the best number to track. Try recording the number of chin-ups you can do, or the distance you can run at a certain speed.
Lower body weight lifting is just as important, if not more important than upper body.
It is important to get a sufficient amount of protein in your diet. 100-150g of protein a day on average is good for keeping strong muscles.