I thought I would personalise my DEVONthink experience by having a rubber stamp (low tech) made to stamp everything I’ve scanned and imported. This means that once my wife and I have finished with the receipts, papers, whatever, we know at a glance that it’s been scanned and can be shredded.
I have a similar system, but not a rubber stamp. All I do is create a small dot with the ink of a pen in the upper right corner of anything I scan. For important documents where it’s not such a good idea to leave a mark, I do it on the back side of the page.