I actually like DiapDealer's approach of only buying a new book when he finished the previous one. It's something I've unsuccessfully tried with paper books, e-books, video games, iOS games, Android games, and Ukulele song sheets.
I'll have to try to muster the discipline, again. Not even for the possible money saving, but to counter the occasionally frustrating feeling of never catching up.
I changed my view on keeping all books on the reader, too. Now that I mostly use the Nexus 7 for reading, I can use Calibre Companion's solid features that make organizing and finding books much easier. I also sat down and learned how to use shelves with the Kobo reader, which also made it more viable for me to keep all my books on the e-ink reader.
It does require more discipline to not flip too much between multiple books, but that is something that can be learned. I hope!