My daughter is shy with children (not adults) and I talk to her about practicing doing things that are a little more challenging. We role play introductions "Hi, what's your name? My name is...", I emphasize how much I love and value my own friends, I attempt to model being friendly, and if we are not within earshot I remind her to say "hello" to friends.
Part of it for her is that she's in her own world, but for truly shy children, gentle, yet constant exposure is wonderful. She may take a little time but like doing anything that we're not comfortable with, practice, practice, practice.
Too, I don't ever call my daughter shy. She may be feeling shy in the moment, but I don't want her thinking she is limited to just shy. I also let her know sometimes I feel shy, sometimes more outgoing, but I think about how my behavior affects other people so I make the attempt to give a "hello."