Well, really there are two ways of making a documentary: one is to find out as much as you can about the people and events involved, write a script and figure out what to shoot to tell the story you want to tell about it, the other is to film whatever happens and try to find a story you can tell with the footage.

Personally the only documentary work I've done has been in the former category, though I've been editing a lot of old 'holiday videos' lately from the time when I used to get VIP passes to see space shuttle launches which kind of fit into the second category. In either case, but particularly if you go the 'shoot then edit' route, I'd say that one thing you really want to do is get as much cutaway footage as possible and not to keep the camera in one place and pointing in one direction at one frame size for too long. You don't want it moving all the time as that will make the footage hard to cut, but equally you want it to move enough that, say, you can take a ten minute shot of one person talking to the camera and cut from wide shots to closeups to eliminate the 90% of the footage that you don't want to use.

Oh, and get a decent mike so you get good sound. You can get away with crappy video, particularly for documentary use, it's much harder to get away with crappy sound.