"How do I mark an action item complete?"
This is a question that I struggled with when I first started using Evernote as part of my GTD workflow.
I tried several methods before I landed on my current work flow.
My first attempt was to put all of my actionable notes in a notebook called "Action Items". I processed and tagged into that box from "Inbox"
The problem was that I would have to re-process that item out of my "Action Items" notebook and into a reference folder or to the trash. Who wants to process the same note twice? Not me.
I solved the problem by removing all of my "What" context tags. I created a new notebook for each of my "What" tags. See my post on Notebook Setup.
The second key to making close-out easier is to use saved searches. See my post on Saved Searches.
With my notebooks and saved searches setup correctly, closing an item is easy. All of my current next actions are viewed in the context-based saved search. For example, when I am in my "Work" saved search and I finish a task, it is a simple, one step process. Delete the "!1-Next Action" tag. Like magic, it disappears from my "Work" saved search. Where did it go? No where. It has already been filed in the appropriate notebook that defines its "What" context.
There is one small loose end to tie up. I created a saved search called "No !When". I set it up to help me find orphan notes that have no "When" context. I run this search a couple of times a week to make sure I haven't dropped anything. The problem is that all of my completed notes now show up in this saved search because I deleted the "!1-Next Action". My simple solution is to select all of those notes that are done and in bulk add a tag called "!Closed". Yes, it is an extra step, but the good thing is that it gives me one more quick look at my closed items to make sure I didn't inadvertently close a note.