Consider the Responsibilities
Being someone's maid of honor is akin to taking on a part-time job. That is, a job that doesn't pay anything and actually costs money. The position often comes with a lot of drama, which even some really great friends would rather not deal with. If you have an inkling that your front-runner belongs in that category, it might be best to make her a bridesmaid instead.
Debbie Downers aside, let's say that you have two or three friends who would be truly honored to serve as your maid of honor. At this point, you should consider each person's "maid of honor" qualities to determine who's best suited for the role. If one of the candidates is well-meaning, but disorganized and chronically late to events, whereas the other friend is type-A to the extreme and loves hosting parties, your choice is pretty obvious. After all, it's vital to choose someone who can reliably perform all of her required duties.
We don't mean to sound impersonal. After all, the maid of honor is also your go-to person for emotional support, and she'll be helping you make some tough decisions, too. Take your friends' personalities into consideration. A strong personality might overshadow your wishes. By the same token, a passive, shy friend might never give you an honest answer.
Whoever you choose, be sensitive to the "runner-up" to avoid or minimize hurt feelings. If choosing between friends is impossible, you can always gauge their opinions about being co-maids of honor.