Decide when you'll speak. Many couples have their officiant follow a traditional script ("Do you, Groom, take Bride to be your wife..."), and then segue into their self-written vows ("Ann and Hal will now share a few personal words").
Think in specifics. When expressing why he's the one, saying "I love how you're always willing to listen without judging" is more personal than "You're nice to everyone."
Avoid getting too intimate. References to his proclivity for pink boxer shorts or how much he loves your off-key singing in the shower is a little too much information!
Consider adding a religious element. If you're Jewish, for example, you could mention the strength of Sarah from the Bible to add a traditional component to your sentiments.
Include a quote from your favorite book, poem or song.
Write your vows separately and keep them secret from each other until your wedding day, which will up the emotional factor. (Practice saying them to a friend.)
Set a 30-second time limit—this will force you to really think about what you're saying and make each word count.