^ I think people forget when they agree to be a bridesmaid, that it is a commitment and it is work. Yes, it can be a lot of fun and very flattering to be asked, but it's going to hurt more when you can't pull through for the bride-to-be than to tell her beforehand that you don't think you'll be able to do it.
And if you've already committed, remember that it is not about YOU, it is about the BRIDE. *sigh* Unfortunately, I don't think a lot of these bridesmaids remember that at all!