1. Lift weights for no more than three to four days per week. Doing so is not only unnecessary but can quickly lead to over-training, especially if you are doing other physical activities such as cardio or playing recreational sports on a regular basis.
2. Limit your workouts to 30-45 minutes and 15-20 total sets. If you can't build muscle and gain strength in that time frame then I'd say you are half assing it. You have to remember that results are greatest when energy levels and mental focus are at their highest. That is during the first 30-45 minutes of your workout. Going beyond that point causes both of these to plummet.
3. Use big, compound exercises and lift heavy. Deadlifts, military presses, squats, bench presses, rows and chin ups should always be the main focus of your muscle building workout programs. These have been the best muscle building exercises since the beginning of time and that will never change.
4. Continually try to get stronger and always track your progress with a training journal. Progressive overload is the most basic but often forgotten principle in weight training. It states that to make progress you need to constantly increase the amount of weight you lift. Follow this rule and you will get bigger and stronger. Ignore it and you will get nowhere. If you are benching 225 right now, you better be benching 315 by this time next year if you want to build muscle.
5. Train with a multitude of rep ranges. Doing this allows you to target both slow twitch and fast twitch muscle fibers and maximizes your muscle building capabilities. Reps from 1-20 should be used to target both fast twitch and slow twitch fibers.
6. Always change your weight training program every 3-4 weeks. After 3-4 weeks on the same program you will start to burn out and your results will slow down. To keep your body in a muscle building state, be sure to change your workouts frequently. If you have been lifting for several years, this may need to be done every two weeks because you will adapt more rapidly to the same stimulus.
7. Make a serious commitment to eating. Proper nutrition plays a huge role in your muscle building efforts. Without adequate calories you will never grow optimally. Force feed yourself if you have to and be sure to time your carbs correctly, meaning around training and at breakfast, while cutting them out at night if you want to stay lean while building muscle.
8. Be sure to get at least 8-10 hours of sleep per day and take naps whenever possible. When you are sleeping is when you are building muscle. Sleep is the time when you recover and grow. Without adequate sleep you will never reach your true potential and your muscle building efforts in the gym may be wasted. Do not overlook this important factor.
9. Utilize recovery methods. Training and eating properly are not enough to ensure the fastest muscle building results. You also have to be sure to use whatever recovery methods you can to accelerate your progress. Some of these include taking contrast showers or baths after training, stretching after training and on off days, icing, using foam rollers and whatever else you can think of to help you recover faster.
10. Find a good training partner. While I left this for last on the list it may, in fact, be the most important factor of them all. Without a good training partner your results will always be less than what they could be. It is imperative that you try to find someone to push you and to compete against if you really want to take your muscle building efforts to the next level.