Thanks everyone for all the info here!

I really like the idea of keeping project planning separate from project lists and next action lists. By all means, get everything out of your head (mind maps, OmniOutliner, index cards, whatever). But I think one of the points of GTD is to trust intuitively that you will know what the correct next action is as the project progresses. I'm realizing that this means putting only true next actions on both my project and next action lists. Actually, in the conventional GTD method, at least as far as I understand it, everyday work is done from a simple list of projects, not multiple project maps.

The benefit of this is that it forces you to reiterate and reevaluate as things change. (I've never correctly anticipated the steps of a larger project from the start). This method also ensures that lists keep refreshing. Once one checks off an action, the trigger would be almost automatic: "what is the correct next action here?" Reviews would then be the time to sync one's lists with one's project reference material (deadlines, necessary steps, mind maps), to make sure nothing has been missed. From my own experience, it's easy to confuse getting everything out of one's head with trying to control everything in advance. The former is the state of flow that everyone raves about. The latter does not allow enough room for adaptation and flexibility. (I'm speaking from my own experience of stale and bloated lists.)

The one exception to this method, I suppose, would be projects with imminent deadlines and a lot of steps. Sometimes, you just need to map everything out and plow through it. But in such emergency situations, I temporarily work from my urgent project list and forget everything else for a while.