Biceps and triceps supersets, in which you alternate biceps and triceps exercises without rest, work really well. A good, basic superset for arms is the barbell curl alternated with the cable triceps pressdown. Take the barbell to the cable machine, that way you eliminate any rest between sets. This really helps to maximise the blood flow into your arms. A lot of bodybuilders have built great arms doing this kind of superset.
The delts are more like the endurance muscles you see with calves or forearms and they can handle more reps. Some athletes who are really gifted can do heavy barbell or dumbbell presses and just grow easily. If you have problems gaining size try giant sets (where you combine 4-5 exercises and complete them consecutively with no rest between). It is really intense and you will feel a killer pump on your shoulders. Do a giant set with dumbbell lateral raises, dumbbell shoulder presses, upright rows and bent-over lateral raises for 10 reps each exercise, or 40 reps to total. If your shoulders are not responding from heavy training, this type of advanced supersetting will really work.
With legs being just about everyone’s hands-down pick as the toughest bodypart to train, physically and mentally, it is better to train them on their own. This bodypart takes all of your energy resources of worked out properly and calculated intensity. Having to go heavy and hard on legs sometimes has bodybuilders passing out or throwing up. It is better to train legs separately, when you can pour everything you have got into them.
Belts are viewed as more of a necessity, simply because your lower back is crucial to everything else you do. There are a lot of people who believe that by wearing a belt, you are limiting your lower back development, and that is true to some extent. But just like with straps, if your lower back is out of shape and you are a bodybuilder, you will want to offer it some additional training exercises with back extensions or lighter deadlifts to make sure the area gets the attention it needs. You are only as strong as your weakest link. Do not leave your belt in the bag because you think it makes you less of a lifter, though. It can be the tool that helps you train safer and for longer than guys who choose not to wear one.
1) Perform an equal number of exercises with the same relative intensity between opposing muscle groups (eg, back and chest, hamstrings and quads, biceps and triceps). If you already have a deficit, try to do more sets on the weaker/disregarded bodyaprt until you have achieved better balance. Then train bodyparts equally.
2) To bring your training into balance, focus on isolation (single-joint) exercises while also modifying the compound movements that target the various muscle groups. For example some exercises, such as the squat and leg press, are generally regarded as quadriceps moves, even though they absolutely involve the hamstrings and glutes. However, by altering foot placement (wider, higher on platforms etc.) you can shift the emphasis to the less involved hamstrings.
3) If you have an existing deficit between opposing muscle groups, spend some extra time stretching the stronger muscle group and strengthening the weaker one.
4) If you find a specific bodypart to be particularly stubborn at responding to an adequate training stimulus, consider adding an extra day (or two) specifically dedicated to working the stubborn bodypart until it catches on.