I wouldn't go there.  

I used to simply raise my arms up and down about 20 times (not necessarily fast) and plus I would rotate each shoulder both forward and backward to loosen up.

I'm not a doctor and I don't play one on the cafe, this is stuff that just works for me.  

Correction:

I simply raise my arms up and down about 20 times (not necessarily fast) and plus I rotate each shoulder both forward and backward to loosen up

The "used to" is still emblazened on my brain because I "used to " be in the same band for many years, but I should stop saying that because I'm still playing regularly!

Freudian slip  

Oh, and by the way....  Smart @$$  

I do a couple kinds of stretches for my shoulders.

First: grab your left shoulder with your right hand, letting your right elbow lay against your body. Then grasp your right elbow with your left hand and gently pull it to your left. You'll feel this in the shoulder and shoulderblade. Always go slow and be gently -- those bobbing stretches they used to show on excercise tv shows can cause injuries.

The second one might be a little harder to describe. To stretch out your right shoulder, traps and upper back, do this: extend your arms straight out in front of you and cross them at the elbows, right over left. Then fold your arms up at the elbow. In time, you'll be able to lock your hands together like this. Take a slow, deep breath and, keeping your forearms parallel to your body, slowly raise your elbows together. Hold. You should be able to feel this in your shoulder and lats. Then lower your elbows, still keeping your forearms parallel your body. You should feel this in your shoulder and traps. This is an excellent stretch that takes some practice to get just right, but should be able to save you lots of grief in the long run.

You can also try bracing your outstretched arm against a doorframe and slowly turning your body away from your arm. You'll get a little of that in your shoulder, but it'll also stretch out your bicep, the flexors in your forearm and your pecs.

Having just spent a lot of time in therapy from rotator cuff surgery, I can recommend a good stretch...Simply twist your arm behind your back, and try to scratch between your shoulder blades.Hold that position for a few seconds, then relax and do it again... Go higher if you can manage it. It also helps if, like me, you have a seriously cute therapist to help!