A very simple, but very effective tip.

I already lost a lot of books (or cds/dvds) by lending them out, and never getting them back. I forget who got the book, and for one reason or another, so does the person receiving the book. Thanks to remember the milk, I've created a list where I note who has the book. No more lost books!

Of course, this also works the other way round. I make notes so I don't forget to return books I lent.