Always remember that determining the true next action(s) of a project is impossible unless you already have at least a basic project plan. Project plans need only be as complicated as the project itself and the planning for it may take 12 seconds (change shower head) or 12 hours (buy car). When I start bigger or more complex projects, my first next action is usually to have a planning session. Which I start by just writing down thoughts and asking myself questions about the project. These are the kinds of things David calls project support materials. They are necessary. To do it properly follow the natural planning model.
After I have at least a basic idea of the project, determining the next action(s) is a matter of tracking how far i've progressed (or regressed, depending) and determining where to pick up. Just like putting your axe away after the day is over. Your axe goes here, the tree stay there, and you leave work behind and go home, with a clear mind. (I hope that analogy makes sense because it sounds great in my head)
Hope that helps!