Well, I've done it several ways. I've used carbonation drops and put them in individual bottles and when I racked to the keg, I racked some beer into those bottles and capped them. I've also force carbonated a keg, and then used this technique to fill bottles as needed: http://www.homebrewtalk.com/showthread.php?t=24678
It's a long thread, but there is a video in there, too, by BobbyM that demonstrates this. It works great, and then you don't need a beer gun.