The first thing I should tell you is that unless a girl already has feelings for you, don't expect much from a simple exchange of paragraphs. With that said, talk about a good experience you had with whoever's year book you're writing that you guys had (unless it's private and personal). Make sure that you bring up a good memory though because she is going to be looking back at her year book and she'll remember that memory and associate it with you because you were the one that wrote it.
Also, what I did was end it with a "keep in contact!" and then leave my myspace or facebook link. I don't like leaving my number.