One thing my DD taught me early on is that social interactions/behaviors are the single most complicated thing we've had to teach her, so I don't have any advice on any shortcuts. It's a looooooooooong process.
Like ashley, DW and I take our role as facilitators very seriously. We do things a little differently, because ashley's list of interventions includes too many structured activities, and the single best vehicle for working out social mores is through unstructured play.
Early in DD9's life, DW and I made ourselves available to play with DD for hours each day. We mostly followed her lead, though we quickly let her know when she was engaging in unfriendly behaviors, and modeled the kinds of behaviors we'd like to see in her. As time went on DW and I began to assert our own personalities a bit more, so DD could learn that different people like to play different games or in different ways, and she had to learn how to accept that.
When she was 3, DD started to play with the other kids in the neighborhood, sometimes at our house, sometimes at theirs. When they were at our home, we played with them sometimes, but mostly left them to themselves, monitored for any signs of conflict, and stepped in as mediators and teachers as needed. This is how we started arming her with the kinds of skills she'd need to deal with people whose behaviors don't match what she'd become accustomed to at home.
Fast forward 6 years, and we're still basically doing the same things mentioned in the paragraph above, in addition to talking about her school experiences and counseling on ways to deal with the situations she encounters there.
This past weekend there were, at various times, five kids hanging out at our house with DD, so something must be going right.
