I used brown cardstock to cut the strips that I'll write the titles on. I cut all of the strips out of brown paper to give the books some uniformity. And voila, a new look for some old books.
While I wouldn't put new covers on all of my books, it's nice to have a pile of coordinating books to add some interest to a sidetable or nightstand. Also, I keep all of the original bookcovers in a plastic storage container so that I can put them back on if I want to.