The secret to all popcorn is to keep the kernals covered in oil and as evenly heated as possible. Microwave popcorn is different in that there isn't a single heating surface like a stovetop, or fire.... So you should always keep popcorn moving over the heat/oil.
I am sure that there are other tricks to doing it over a campfire. BrenR?
[EDIT: Just saw that BrenR replied while I was typing mine.]