I use keywords for multiple purposes.
To address your task of keeping track what you have published:
Set up a Collection Group named Published
Make a new keyword named Published
Make Collection Group your Target Collection.
Add images to that Collection.
When images are added go to the Collection, select all and click on Published in the Keyword List.
You could add the keywords before or immediately after you publish them. You could skip the keyword step if you are diligent about moving published photos into the Collection.
I also use keywords for editing (not processing). I have Proofs and Candidates. Proofs are edited to select Candidates. The Candidates are used to assemble portfolios, slideshow galleries or submissions.
I use flags the way you use stars. Proofs or better are flagged, everything else is unflagged, and technically flawed (unusable) images get the X and are eventually deleted. Because disk space is inexpensive, I treat the Library Folders as virtual contact sheets. I never delete in-flagged images.
I also back up the original raws to an external drive automatically upon import to LR. So everything on the card is saved in it's native format. That drive is backed up nightly to a second external drive. Upon exit the LR Catalog is backed up to the first external drive. This means I have four copies of the Catalog (internal HD, Time Machine HD, external HD 1 and external HD 2).
I use stars several differant ways... which is not a good idea. I use stars to rank flagged photos. This is done for all flagged photos which always have at least one virtual copy. When I'm editing (not processing) I use stars to separately rank the photos for the project, portfolio or series I'm working on. Any thing that gets published (in the LR context) has four or more stars. Five stars means I that image is finished and requires no more attention. I use colors to indicate different versions of final or close to final images. My method is inconsistent.
I always make new virtual copies for each new editing session.
