I am think about add a shoulder holster to my carry rotation.
I do not have one or have ever used one.
What things should I look for or should avoid?
At one time I thought I read about a holster that one could wear under a shirt and not print.  or print much.  
Any ideas on a holster that one could wear under a long sleeve shirt?
thanks for any advice.

First and foremost it is important to remember that having it with you is better than it sitting in the safe at home.  I use a spandex band to carry my Bersa .380 on my ankle when I am dressed up.  I cannot draw it as fast as from my hip but at least I can get to it from behind a barricade if necessary.  That said, I have used several under the arm shoulder holsters and the question for you is if you (a) don't care if it shows and (b) know what is most comfortable for you after wearing it all day.  I personally like the double holster undershirts that do not print.  The problems are that they do get wet with excessive sweat and they are expensive but buy two and keep one cleaning all the time.  If not that, then a black and a white soft nylon type.

Try the Alesi bodygaurd

Depending on my needs, I use both a Miami Classic and a Blackhawk Serpa shoulder rig.

Ted Blocker Life Line shoulder rig

I like them more than some people do. But it's not something you can do halfway or even easily; they take a lot of adjusting and tweaking to make them work to their optimum. Also, some (most..?) just don't work well as far as serious concealment.

This thread has some good discussion: http://www.ar15.com/forums/t_5_10/164048_shoulder_holsters___anyone_actually_use_one_often_.html&page=1

Has anyone tried one of these?
kangaroocarry.com