Addendum to my earlier post above:
I've found myself in the past several months using another solution to "tickle" items. I found that as I got free of my coursework for my graduate program, I would forget where my days went and be very hard on myself even though in fact, I might have gotten quite a bit done that day. So I started keeping a daily logbook of my bigger completed NAs (i.e., not "water plants" but certainly "write method section of dissertation proposal"). I also use it to manage my use of the Pomodoro technique for some of my longer NAs--I mark my pomodoros in the book.
Since I therefore now have a dated page for each day (I made my own notebook to see if this would work for me, but now that I've learned that it will, I bought a Moleskine Daily planner for 2013), I've found myself writing down future NAs in the log on the day they need to be done.
It's working very well, so far. This is the thing I find most interesting and useful about GTD--you keep adapting it to suit however you're working currently.