I am constantly late at returning library books. While studying at the University of Waterloo I racked up huge fines, often just by being one day late (they would send me an overdue notice the day after they were due, thus incurring $1 per book, before I could renew). I eventually wrote a little python script that would log on to the University of Waterloo's library website and renew books for me (the script ran automatically every night). That worked great for the last year that I was at Waterloo.
When I came back to Vancouver I was also late at returning books to the Vancouver Public Library. I stumbled on a site a while ago called Library Elf that sends me reminders when my books I have taken out from the VPL are almost due. It supports many other libraries as well. It automatically logs into any libraries that you add to your account and you can receive notification up to one week in advance, or every week, by RRSS and/or email. It has worked great for me, except for one time when it didn't send me any reminder. Perhaps GMail thought it was spam, I'm not sure. I might add the RSS option, just in case.
Another neat feature is that in the reminder email, they will suggest a book that they you might be interested in, based on the books you have borrowed. It just recommended a great authentic Mexican cookbook to me that has a 5-star rating on Amazon on 13 reviews!