1. Not Counting Calories. To get big you need to eat more calories than you burn. If you don't do this simple, most important step, you won't gain weight. If you aren't gaining weight and think you're eating as much as you can, count your calories and you'll see that you're probably eating a lot less than what you think you are.
2. Not Eating an Adequate Post Workout Meal. After lifting heavy weights, your body's number 1 priority is to get your blood glucose levels back to normal. This is why you need a good post workout meal. The best kind of meal after working out will be digested quickly. It should be high in simple sugars and protein while low in fat. Sugars and whey proteins are digested quickly. They will help your body return to a normal glucose level while also providing your muscles with a quick dose of protein to begin the recovery process. Shakes are better than whole foods because they can give your body the nutrients it needs faster by being digested quicker.
3. Listening to the Wrong People. Getting big requires that you learn how the body works. The mistake a lot of people make is listening to advice given by people who look good. Most of the time, the 'big' people don't know right from wrong. They're convinced that what works for them will work for everyone. The truth is that not everyone's body works the same. What worked for me might not work for you.
4. Overlooking Certain Muscle Groups. There are some muscles that are overlooked by guys wanting to get big. Don't be one of these people. To get big the right way, you need to workout all the areas of your body to look proportional. You don't want to have a huge upper body with sticks for legs. Some common overlooked muscles are forearms, lower back, abs and legs.
5. Using too Many Supplements. There are hundreds if not thousands of supplements you can buy that promise to increase the size of your muscles. There are some that are worthwhile but the vast majority are a waste of your money. Remember, the most important rule of getting big is to eat and workout enough. No amount of supplements can replace this basic foundation.