Indeed the muscles of the deltoids and trapezius are extremely important when building one's body. When fully developed they form the " X-frame" of the body whereby the wide deltoids taper into a narrow waist which flares out to sweeping quads. Well developed shoulders are one of the main ingredients to creating an aesthetic physique.
The most aesthetic bodybuilders such as Steve Reeves, Flex Wheeler, Ron Love, Shawn Ray, Chris Cormeir, Kevin Levrone, Bob Paris, and Lee Labarada all have tremendous delts and traps. Good shoulders can improve or even mask flaws on a bodybuilder. Case in point is Ronnie Coleman. Ronnie doesn't have the narrowest waist in the world but his delts (along with his lats) are so damn big that his V-Taper seems amazing. The same goes with bodybuilders like Gunter Schielerkamp and Paul Dillet. Athletes outside of bodybuilding also have amazing physiques because of their shoulder development. Boxers like Felix Trinidad who, weighing only 160lbs has coconuts for delts, as do football players like running back Eddie George. Wide delts are even evident when you are fully clothed; tailors have designed suits and coats wit more padding around the shoulder girdle to enhance the V-Taper of the torso. The shoulders are also evident in almost ever pose you strike onstage especially the lat spreads, back double biceps and most-muscular crab shot.
OK NOW YOU KNOW THE DELTS ARE IMPORTANT,
NOW HERE'S HOW YA TRAIN EM '.
In their simplest terms the shoulders or delts have three sections:
1) The anterior or front delts tie into to the pecs and biceps.
The front delts come into play during all pressing movements for chest such as bench presses and incline presses not to mention flyes. They also come into play when you are doing biceps curls if you are using a heavy weight that warrants a cheat motion.
The front delts are worked the most intensely during overhead pressing and directly with front raises. Overhead pressing motions are the basic compound movement for delt development and they target most of the stress onto the front and side delts. To keep stress on the delts take a medium-wide grip at about three inches outside your shoulders and press to just short of lockout to help keep the triceps out of the movement. Presses are a basic movement, ideal for heavy weight in the range of 6-10reps. There are many pressing exercises to choose from.
Standing Barbell Presses are pretty much one of the best exercises not only for delts but for the entire body as a whole. It recruits the triceps ( just as all pressing exercises do) in addition to the lower back and glutes for stability. Since so much stability is involved in the exercise I think it is ideal for an intermediate trainee who has mastered technique. The standing press is a good movement if you are training for more strength and power since you use so many muscles at one time. I urge you to where a belt for lower back stability on this movement.
Seated Barbell Presses or Military Presses are another good basic movement. These can be looked at as a better alternative to the standing press since you can concentrate on contracting the front and side delts more than on the standing version. The old school method was to bring the barbell behind the neck but in recent years this technique as come under scrutiny because of the risk of shoulder impingement.
Dumbbell and Machine presses are also very good choices for delt mass. Dumbbells allow for a longer range of motion as to certain machines. Although machine movements do not call upon stabilizing muscles as much as free weights they are still good for development since they allow better concentration on the muscles you are trying to work. Plus, do let people tell you machines don't build size-- Lou Ferrigno and Arnold relied heavily on Smith Machine Presses and they had amazing deltoids.
2) The lateral a.k.a side delts
These are typically is considered the most important of the delts heads since it contributes to shoulder width the most. The side delts respond the most to lateral raises which is considered and isolation movement (although you never can fully isolate one head of the deltoids since they are all part of the same muscle).
When doing lateral raises you should aim for a strict range of motion and to keep a slight bend in your elbows--if you are using a slight cheating motion you can bend your elbows a bit more but I suggest you only do a few sets in this fashion. Additionally keep your elbows higher than your wrists when raising the weight better recruits the side delts because the traps are called upon less. Another tip that Arnold used to do was to keep your palms parallel to the floor with the wrist in a slightly supinated position as though you where pouring a pitcher of water and this technique helps keep stress on the lateral delts rather than on the front delts. In addition to lateral raises, Upright Rows with a wide grip are a very good movement though they tend to involve the front delts and traps as well.
There are a few good variations on lateral raises. You can do them one arm at a time which affords more concentration on the delts and helps cut down on cheating. For a super strict movement you can do lateral raises on a steep incline bench-- these where a favorite of Arnold. Plus you can do side laterals with a cable, a strict movement favored by Kevin Levrone.
3) The posterior or rear delts are extremely important.
Not only do the represent a large piece of the delts, the also are a large part of back development. Look a twisting back shot of Chris Cormeir or Arnold and you can clearly see how much their development contributes. The posterior delts are worked whenever do draw the scapula back as you do in rows, pulldowns, dealdlifts and chins. To help isolate stress on the rear delts for reasons of shoulder development, bent-over lateral raises are one of the best movements. Here you should make sure to keep to bend your elbows slightly and to pay attention to raise the dumbbells so that they are in line with your collar bones; if you don't then the middle traps and other back muscles will complete the movement and the delts will be left under worked. Many people place their rear delts last in their routine but I discourage you from doing this since the muscle is so important for over all development.
My favorite version is standing bent over laterals but you can also do this exercise seated on the end of a flat bench. For a very strict and productive movement you can do prone bent laterals in which you lay face down on a low incline bench--this eliminates cheating and hits the rear delts like crazy. Pec-Dec stations can also double up as rear delt flye machines and this is another good movement. Cables are also very good to use from time to time since they allow you to use continuous tension on the rear delts, not affording them any rest during the set.
- Standing Lateral Raises (sometimes with one arm) 3-5 sets of 8-12reps
- Seated Smith Machine Press 3-4 sets of 6-10 pyramding the weight
- Standing Bent Lateral Raises 3sets of 6-10 reps
- Wide Grip Upright Rows 3 sets of 8-10
After implementing my tips I am sure your delts will be more than just boulder shoulders...they will be freaktoids!