All 5 phases are important but the 2 critical things I find that I *must* do for peace of mind are:
1) Have a UCT (Ubiquitous Capture Tool) on me at all times. I think nothing of writing down dozens of ideas a day. Big, small, important, frivolous, pending, future, current, it doesn't matter. If it's something I've got my attention on for more than a few seconds, into the UCT it goes. When it comes time to process them (minutes, hours or days later) I can decide then whether or not it's really important and if so, where to put it in the system. In my opinion, this is the critical step for having a 'mind like water'. When something starts nagging at me it just takes a couple seconds to whip out the UCT, jot down a note and be done with it. From that point on it's 'In the system' and I can relax and concentrate on the work (or play) at hand.
2) Do the weekly review to close those nagging, open loops. Don't forget, part of the weekly review is an 'empty your head' brain dump. Periodically, I like to use trigger lists to make sure that this brainstorm session is thorough. In your case, don't even wait for the weekly review. Get some trigger lists and start working them.
Reducing the number of inboxes has helped me a lot, too. Knowing that all the stuff you need to process is right in front of you makes it easier to know when you're really and truly finished.