Most often, I check it as done. (Performing a search will return items that are completed as well as archived items in Outlook).
There are times when I replace the action just completed with the next action. This is particularly true if I have information in the note section (notes from phone call regarding the completed action) that I may need.
If the item appears to be something of "lasting value" (again, probably because of documentation in the notes section), I drag the task (in Outlook) to the "Notes" icon and add the date as the very first thing in the first line of the note.
I delete. Unless you are in a business environment where documentation of all your 'done' activities are required, just delete and move on to the next action. What you have already 'done' should be self-evident...a done action is reminder enough for me.
Getting buried in self-reporting is a form of water torture. Make things easier on yourself and your system by deleting items that are completed.
I mark it done and eventually archive it. This is for historical/reference purposes -- if I ever need to look back at discussions, agreements, decisions, confirmation numbers, etc, -- I can do so easily.
The main reason though, is the satisfaction I get from it. Being able to look back through the day/week/month and seeing everything I've accomplished makes me feel productive... as if I'm getting somewhere.
I delete them at the end of the month but before I do so I scan them all to
1) like Kathy feel 'proud' that although I felt like I wasn't getting much done that I actually DID achieve a lot
2) also like Kathy, occasionally some vital piece of information slips through the cracks - someone's cell phone number that didn't get entered from a new business contact etc - the name of someone in a store that I didn't note in my contacts but is on a completed to do....etc...
Just occasionally I've been glad they're there.....