Think before you pick:
Once you ask, that's it. You can't change your mind once you've asked them. Be sure to not ask too far in advance and know once it's done, it's done.
Include your siblings:
Family is forever. You may eventually lose touch with your college roommate, but your sister will always be your sister.
Don't include someone just because they were in your wedding.
My 18 year old cousin was our flower girl. Why not break tradition and include people who are important to you rather than just who society tells you you have to include.
Research other roles:
I am having a very small wedding party, but still want my other friends and family to be included in other ways throughout the wedding. Ask someone to do a reading or be an usher.
Ask people who are going to support you:
At the end of the day, you're the bride. Ask people who are going to want to be there and love you no matter what.