There's some good advice here, some less good advice, too.
I think it's going to be difficult to seperate the useful advice from the less effective stuff. I'll add my own advice, and hope it those that take it find it useful.
First of all, "wide shoulders" are a relative term.
Shoulders that are wide for a fellow that is 5 feet tall will look narrow on a fellow 7 feet tall. The taller you are, the broader your shoulders need to be to look wide.
Also, the appearance of width depends also on how thick a person's waist is. Given the same width shoulders, a person with a 28 inch waist will give a better impression of width than a person with a 36 inch waist. Trim down your waist size, and your shoulders will look wider, even if they aren't.
The way to actually make yourself wider is to build your shoulders (Deltoid muscles), specifically the lateral head. You CAN isolate the lateral head, but isolation exercises (like dumbell side raises)tend to limit you to light weights, and light weights do less to stimulate growth than heavier weights do.
The heaviest exercise for the shoulders directly would be the seated barbell shoulder press in front of the body. The standing press is good, but the disadvantage of doing the exercise standing are twofold:
1. Your body is less stable standing than sitting, so your nervous system is busy dealing with the work of keeping you balanced on your feet in addition to actually lifting the weight, potentially reducing the poudages
2. There is the added problem of the tendency to add momentum by bobbing your knees and hips to get the weight moving (cheating)
The standing press has its uses, however. The extra work of balancing the weight is useful for athletes that need to exert force from a standing position (football players, boxers, shot putters), and the body learns to integrate the use of the legs, hips and waist into exerting force and channeling it through the arms. It is also useful to use the legs to get the more weight than you can lift in a positive contraction in order to do negatives (An advanced technique that I will not get into)
So, SEATED BARBELL PRESSES is the answer. As to how much weight you should use, make sure that you work with a weight that enables you to use a full range of motion, namely lifting from around clavicle level to full lockout.
Research in many textbooks has shown that a repetition range of 8-12 reps is ideal for muscle hypertrophy (growth). Less than 6 reps is more effective for building strength (Less growth), and more than 15 reps is effective for building local muscle endurance (Lactic acid tolerance). I recall reading somewhere that less than 5 per cent of strength trainees can do a seated press with their body weight.
For an overall growth stimulus, however, it is useful to stimulate a full body anabolic hormone cascade. That is where exercises like squats and deadlifts come in. In order to stimulate the release of testosterone, as much muscle in the body as possible must be recruited to work at the same time on a movement, and these two exercises use more muscle mass simultaneously than any other exercise. Start out with squats and/or deadlifts with about as much weight as you can reasonably handle for the rep range suggested, to get the anabolic juices flowing.
Any lat work to get the v-taper going will help create the illusion of width, as well, without directly contributing to shoulder width. If you flare out your lats, if they are well developed, they will naturally expand and push your arms and shoulders out (but not, generally, when you are relaxed). The best way to develop your lats would be through chinups (Weighted, assisted, whatever you can handle) and, strangely enough, the aforementioned deadlifts. The function of the lat muscles are to bring the arms from front to back (Flexion), to bring the arms toward the midline (Adduction) and, although few people know, to extend the lower back (or maintain a back extension position)
Although big strong traps are very useful in protecting the neck and for expressing total body strength, overdeveloped traps can actually make the shoulders look more narrow.
Good luck.