As a player I am generally one that tends to accidently roll over the quieter players, so when a new person joins the group I tell them "If you want to do something and we are accidently skipping over you, don't be afraid to kick us to do something you want to do." Everyone laughs and it seems to make a good ice-breaker for the new person. If it seems we have a shy player, and I notice it (sometimes I am as thick as a board, sorry) - I try (hopefully) to make certain the shy person gets a chance to do things - I'll ask "Hey Sally (or Sam or whatever) is there anything you want to do in town while we are here? Anything you want to buy or check out? Sometimes it seems the quiet ones get missed when it is not an "encounter" situation and the GM isn't keeping track of whose turn it is to attack/cast/shoot.
When I am the "new" person in a group, I tend to hang back a bit the first few session until I see how the group interacts with each other as players and as characters. I do speak up if there is something in particular I want to comment on or get done, but it takes me a little bit to get into my comfort zone and learn where I can make jokes in game and out of game; or push the other characters a bit if we seem to be bogging down on something. Doesn't mean I am shy, just means I am trying to figure out where I want to fit in.
Most of the long running groups I have been in occassionally had pot-luck, barbecue, or order chinese take out sometimes. Some days we might all get together to go to a movie we all want to see. I think that this could give the shy person a chance to learn more about the people they are playing with, and might make them more comfortable with the other players. Plus it's added fun!