Amazing Delts!

The Deltoid is a large, triangular shaped, 3-headed muscle which goes from the clavicle and the scapula at the back of the shoulder down to where it inserts in the upper arm. The basic function of the deltoid is to lift and rotate the arm. The 3 heads of the deltoid are the anterior (front), medial (side), and posterior (rear).
During the 1940's men often wore coats with pinched waists and huge, padded shoulders, to give them that "V" shape look. Coincidentally that's the look Bodybuilders work very hard to develop, and a major part of this look is wide, and very well developed shoulders.
One of the first bodybuilders to develop the classic V shape was Steve Reeves. He was able to create that look because of naturally wide shoulders and small waist. Shoulder width and deltoid development are two different things. An example would be Steve Reeves, who in spite of his width, was not particularly thick in his delts. On the other side would be Larry Scott whose narrow proportions were offset by his massive shoulder development.
Training the Deltoids
There are two kinds of exercises for the deltoids- straight arm raises, and presses.
Raises
Raises involve lifting the extended arm upward in an arc. Raises don't involve the triceps, but almost completely isolate the various heads of the deltoids. When doing raises do them really slow and controlled.
Presses-
In presses you begin with your arms bent, the weight around shoulder height, and press the weight straight up over head. Because you are straightening your arms, presses involve the triceps as well as the deltoids. At the top of the movement stop just short of lockout to help keep the triceps from doing too much of the work.
Arnold presses -
This is my favorite deltoid exercise, and has helped me add a lot of size and width to my shoulders. These work both the side and the front head with a longer range of motion. I started doing these 3 months ago and have seen amazing results. Start with the dumbbells up by your shoulders where they would be at the top of a dumbbell curl. press the weights up overhead-not quite to the point where they lockout, while rotating your hands so your palms face forward at the top of the movement. hold for a moment and lower the weights and rotate your hands back to the starting position.
Try to include a compound movement first, like barbell presses, and be sure to include laterals with dumbbells and or cables to hit the side head and bent over laterals for the rear head. Some other good shoulder exercises are Clean and press and smith machine presses. The smith presses should only be used for variety every once in awhile and should not replace the mass building barbell press. You can also include heavy upright rows in your shoulder routine which involve the traps as well as the shoulders. Cable laterals may also be used in place of the dumbbells.
Since the front deltoids are usually not a weak area for many people because they are involved in many other exercises, I have not included and front raises. If you do have a problem with them then I recommend adding in a few sets of front raises at the end of your workout. Also the rear deltoid which a lot of people have trouble with, may require some extra work.
Training the Traps
I have decided to include a little section on traps in this article also, since I, as well as many other people choose to train them on shoulder day. I like to get all the shoulder stuff out of the way, since the traps get used in a lot of other exercises too, then add in a few exercises for traps to finish off my workout. Since the traps get some work from the other shoulder exercises I find that 6-8 sets is enough.
Shrugs
The best exercise for the traps is the shrug. The basic barbell shrug is still the number one mass builder for this muscle. Start by holding a barbell loaded with lots of weight, about shoulder width apart. Now lean in a little so the bar doesn't touch your legs and try to touch your shoulders to your ears. Shrugs may also be done on the smith machine or with dumbbells.
Upright rows
Another exercise for traps is upright rows. I use them for variety sometimes, but i mainly stick with shrugs since upright rows don't stimulate as much muscle and the attention is divided between the traps and shoulders. These can also be done on smith machines or with cables.