Thanks for the love! If you are set on Evernote, check out my post on GTD in Evernote With Only One Notebook. You can find it here: https://www.40tech.com/2009/08/25/getting-things-done-gtd-in-evernote-with-only-one-notebook/
That method works well, but relies on a list of tags and nested sub tags. The problem there is that the sub tags will do the same thing in mobile that notebook stacks do — i.e. Revert to alphanumeric order, structure be damned. The good news is, I believe Evernote is working on an update that will fix the problem, at least with Stacks.
Another option you could try is using AwesomeNote as a front end for your mobile setup — if you are iOS, that is. See this post by one of your fellow readers: https://www.40tech.com/2011/04/27/gtd-on-the-go-with-awesome-note-and-evernote-reader-workflow/
I use the Producteev method now, as I needed Google Calendar integration, reminders, and some visual blackboxing (see: pretty) to combat task overwhelm. It just works better for me, at the moment. The Evernote method (without a front end like AwesomeNote) is really more for those who are want to combine a pen and paper style system with portable technology. I also have it on good authority that subtasks are on the way…