I started with the bed around 3 or 4 mos - I would nurse him down and then quietly extricate myself. Then at around 9 months, I just started trying to get him down for naps in his crib. I would nurse him and get him to be tired, but not asleep. Then I would lay him down and pat his back. If he fussed, I would pick him up and rock him. It took a lot of practice and patience, but he got the idea. Now we can lay him down in the crib pretty easily.
He still cosleeps at night, though we are talking about trying to get him down in his crib at night (he's 14 mos now). We're total wusses about moving him from the bed, though. Mostly b/c we don't want to have to get up and go to another room at night, but also b/c it seems strange to have him so far away.