While doing wikipedia/osm workshops, I make it a point to start with the "Why am I doing this and what do I gain by being here" section. This lets everyone understand what the open movement and community is about, and what I personally gained/gain by being a part of it. After this, I let everyone know that those who would like personal attention and help can move to the front so that its easy for me to attend to them.

Personally, this has worked very well and also made people more interactive rather than become a classroom type session.