I actually do 6 shoulder excercises a week...

Barbell Shrugs (4x10) Shoulder Press machine (4x
Dumbbell Shoulder Raises (4x10) Behind Neck overhead press (4x
Tightgrip Pulldowns (4x10) Sitting Row Machine (4x

But honestly, you could do a lot less and still keep your shoulders well built and strong. A good shoudler workout just needs shrugs as long as you do about 8-10 good sets. Prop 4-5 with dumbells, and 4-5 with a barbell behind your back. Just make sure when you do them that you basically make a small circle with your shoudler as you pull it up and try to hold it at the top for about 2 seconds. I choose to do the other excercises to help build better stabalization muscles in my shoulders, not so much as bulk. I have also found that over head stuff really helps all my lifts.

And yea, when I do shoulder raises I do one arm at a time, bringing them to a little over head level with a straight arm. I usually use 25's but if Im pumped up I have used 30s a few times. Im really concentrating on form right now. Each movement slow and precise, working very specific muscles. I only wait 30 seconds between sets, and 2 minutes between excercises, so it really is an intense hour and 15 minute workout.

For me, I like to use like a 8 week power lifting program, where I concentrate on breaking records and am a little more leniant on diet and form while lifting. Then I take a week off, and go to a 10 week bodybuilding routine where I concentrate on form, and choose one body part I really want to put extra effort in. Like for me now... its my back and abs. I was born with a naturally big chest and shoulders, and I worked my legs to death on my last cycle.. so now I want to really punish my back and try to get that big V. If you have any more questions let me know... Ill be glad to help you out.