Here are a few tips from About.com…
1. For friends who live out of town, There's nothing wrong with calling her, but I think it's nicer to take the opportunity to write her a card. There are now will you be my bridesmaid cards, but you can also make one, or use your stationery to write a personal note.
2. Ask Her With a Sense of Humor It's a long-standing joke that bridesmaid dresses are ugly. The people at UglyDress.com have assembled a mind-boggling collection of bad bridesmaids dresses. Print out your favorite ones (if you can stop laughing), and use them to creatively make a card. You can write inside something like "I know you've always wanted to wear a peach monstrosity with a butt-widening bow on the back. Will you be a part of our wedding party?" or "I will never ask you to wear an ugly dress, as long as you're my bridesmaid!"
3. Send Her a Book Wrap a bridesmaid guidebook in pretty paper, and put a note on top saying "This is something I thought you'd be needing soon." Be sure to tuck a personal note inside the book before you wrap it. She'll have the surprise and laughs when she first opens the gift, but still be able to read your touching words.
4. Picture Yourself in a Celebrity Wedding Search for pictures of iconic celebrity weddings like Liza Minnelli's, Britney Spears, or even Charles and Camilla. Cut out one of the bridesmaids' faces and put your friend's picture there instead. On the inside of the card, write something cute like "I know our wedding can never compete with your pal Liza's bash, but at least you won't have to wear such an ugly dress, or put up with the paparazzi. Will you be my bridesmaid?"
5. Make a small photo album of pictures of the two of you. On the last page, write your wedding date, leaving room for a photo of the two of you. As she's looking through the book, and gets to that page, say "I can't wait to put a picture of the two of us from my wedding day on that page. Would you be my bridesmaid?"
How did you ask her?