Tip 1: You need to eat big bunches of nutritious, whole foods every 2-3 hours. There is no magic supplement or magic pill, powder or juice. Despite what supplement manufactures want you to think. You need food. Stuff like Tuna sandwiches, beef steaks, chicken, pasta, rice with beans, natural peanut butter and other nuts.
These methods are good for the person who is naturally thin and wishes to gain mass. Take a wide range, but even more importantly, make sure that you eat often and with consistency. Most people simply don't eat enough good food to get bigger muscles.
Tip 2: Make sure that you track your workout progress by keeping records. You always want to progress. Progress can come in many forms. If its a few extra pounds on the bar that is good, if its a few more reps each training session, that is good too. If its 10 seconds less per set, that is good too!
Tip 3: You need to push yourself hard but not for long. So make sure that your training time is low but your effort and output is high! Your time in the gym should be an hour or less. If your time in the gym is longer than one hour, chances are that you are making more friends and less progress. The amount of weight that you should use must be challenging and when this is the case, you should be out of energy by the one hour mark. If the weights you are using are tiny and your training session is long, you are becoming good at endurance, not at building muscle.
Tip 4: Choose exercises that use your entire body. As long as you have decent movements and have no previous injuries, these must include squats, dead-lifts, presses, rows, chinups, and perhaps some Olympic lifts.
These force your body to have an overall growth stimulus. Bicep curls and tricep kickbacks are fine for those guys who are ALREADY big. However these are not the way that you are going to help yourself get bigger.
Tip 5: Train your whole body and that means legs guys! Too many guys just train their upper bodies only i.e. arms, shoulders, etc... neglecting their legs and back muscles. this is unhealthy for your joints, and looks ridiculous.
Tip 6: You do need to change and mix up your rep range and the number of sets that you perform. Sometimes use five reps for five sets, sometimes use eight reps for three sets and sometimes three reps for eight sets. Keep the reps under 12 and lower the sets as your increase the reps per set. The lower the reps, the higher the res periods (2-3 mins) and the higher reps sets need shorter rest (1-1.5 mins. You can also vary tempos i.e. lift or lower the weight faster or slower because this is a kind of variation as well.
That's it! Go out, train hard, eat smart and build muscle.