I "lifted" (get it?) this a while back from another board; it's a decent read with some valid points. MMA777 was the original poster.

Bulking Up Mistake #1. Not eating clean food

The top mistake people make when adding mass is reaching for any food that’s within an arm’s length. They use bulking as an excuse to indulge in all of the desserts and fast food they avoided while competing and getting in-shape for the beach. After all, you’re trying to add weight, so why not eat a little calorie-rich fast food in the off-season? It is true that you need to get plenty of extra calories during the bulking phase to add weight. However, you still need to be watching what you eat, and this means consuming lean meat and healthy fat sources rather than super-sized number 2 meals.

Bulking Up Mistake #2. Not eating enough... period

If you want to grow, you have to eat. It’s that simple. This being said, there are still bodybuilders who don’t get enough food when they’re trying to add mass – this is especially true of women who want to keep their contest form all year long. After dieting and getting a great physique, many people just don’t want to put weight on because it also means adding fat. But you can’t worry about fat during the off-season since this is the time when you need to be making major improvements. As long as you’re eating enough clean healthy foods, you won’t have to be so self-conscious about putting on a little fat.

Bulking Up Mistake #3. Skipping out on cardio

Cutting out cardio would make sense while adding mass since you don’t want to burn too many calories. But the problem with not doing cardio during the bulking phase is that your appetite will go down as a result. That’s why you should include at least three, 20 minute cardio sessions in your routine each week. Cardio sessions keep your appetite up and have you craving more of those clean, healthy foods. Besides having a positive effect on your appetite, cardio will also help you pound out those last few reps. Having a good cardiovascular system enables you to lift more without getting fatigued too quickly.

Bulking Up Mistake #4. Overdoing Cardio

On the same scale as not doing enough cardio when adding mass is overdoing cardio. This typically happens when bodybuilders continue their heavy cardio schedule into the off-season. The major problem with this is that it will be nearly impossible for you to add a significant amount of weight during the bulking phase. That being said, you should still stick to the aforementioned plan of doing at least three, 20 minute sessions of cardio a week (if not a little more). They key here is to strike a balance so you keep your appetite up without burning too many calories.

Bulking Up Mistake #5. Cutting out carbohydrates

Most bodybuilders are smart enough to see past the fad diets, and know better than to cut carbs out. However, many people still don’t get enough carbohydrates when trying to add mass. And this is a major mistake since carbohydrates are an excellent source of energy that can fuel your entire day. Going even further, carbohydrates can drive more glycogen into your muscles and aid in protein synthesis after a workout. With everything that carbs can do for you during the bulking phase, it’s very important to not skip out on them. Make sure to include plenty of carbohydrates in your breakfast to really add some fuel!

Bulking Up Mistake #6. Not setting up clearly defined goals

Every bodybuilder has their weak points and muscles they need to development more. Unfortunately, many athletes either avoid putting more work into these muscles or just aren’t aware of the problem. If you’re not aware of your weaker areas, then you should talk to judges at competitions and/or ask fellow bodybuilding friends for help. On the other hand, if you’re just avoiding these body parts because they’re not your favorites, you should make it a primary goal to add more development to them.

Bulking Up Mistake #7. Mis-managing your meal times

The best way to setup an eating schedule is to spread out your meals every 2-3 hours so your muscles are getting enough protein throughout the day. This being said, there are still people who will skip meals and either try to make up for them by eating double the next time, or just not make up for it at all. In either case, your body will start going after the amino acids in the muscles when not fueled every 2-3 hours; overeating every 5-6 hours is not going to prevent this either. Since you can’t expect to replace lost meals, you’ll want to make sure to try and at least eat something every 3 hours to prevent muscle wasting. Even if you’re not hungry, at least eat a snack to keep your body from feasting on the muscles.

Bulking Up Mistake #8. Overusing Machines

Machines are an excellent way to add some extra definition to your muscles at the end of a workout. However, machines should not be used as the primary means to add bulk since free weights are much better at accomplishing this. Free weights allow your muscles the maximum range of motion and recruit the most muscle fibers. The free weight movements you’ll want to do include presses, deadlifts, squats, rows, and curls. Machine-based exercises are fine to do at the end of a workout, but make sure that you’re not throwing them in right before the tough, compound movements.

Bulking Up Mistake #9. Letting the scale dictate your diet

When your main goal is to add weight, putting on extra pounds can become addicting. It can become so addicting in fact that some people will weigh themselves every hour. When this becomes the case, you may snack and overeat throughout the day just to see the scale go up. And the major problem with this is that you will also put on a great deal of body fat too when you’re eating just to add weight. After all, your body can only add so much muscle mass before the rest starts turning into fat. So instead of focusing solely on weight gain, you should be focusing on what your image looks like in the mirror. But make sure not to get carried away with checking yourself out!

Bulking Up Mistake #10. Working out too frequently

Since bodybuilders love adding mass, they tend to overdo the training and don’t give their muscles enough of a break in between workouts. When this happens, no amount of dieting, sleep, etc. will help your muscles recover. Muscles need time to fully heal in between sessions since they get broken down during workouts. Assuming you’re workout out hard enough, a muscle group will need about 3 days to recover; possibly longer if the muscles are still sore. So always make sure to give your body adequate rest before hitting the gym again.