She's someone who you're sure will be in your life forever.
She's not a girl you just met two months ago and clicked with. Or someone you don't know that well but she's really popular and you'd be honored to have her standing up for you. Instead, the best MOH is someone you've been close to for a long time and who always has your back and always will.
She's not a drama queen.
Or a diva. As you know from friends' weddings, there's usually someone who gets overly emotional or needy but, if anyone, that should be you, not the MOH. You need someone who will stay calm when the limo is late, the flower girl is crying and your nerves are shot. She's supportive and knows she's there to help, not hinder, you. The last thing she'd do is call attention to herself, even if her shoes are (ouch!) too tight.
She takes charge of the other girls, texting reminders about dress fittings and shower prep get-togethers. If the group has a problem with something you've done (shocker!), instead of each one whining to you about it, they tell her because she's the de facto leader. And she's the one who will gently but firmly tell you that, no, the bridesmaids will not all be wearing their hair in a bun, as you requested, since some have super short cuts or don't particularly look good in a bun. Down, Bridezilla, down.
She's your sister.
But this isn't an immediate slam dunk just because you're siblings. If you're closer to her than you are to anyone else, go for it. But if you're chummier with a girlfriend, pass over sis for your pal. Mom may not be pleased but you should do what feels right, even if it means your sister is relegated to bridesmaid.
She's someone you don't "owe."
Being chosen the MOH should come from a place of love, not obligation. Maybe you were her MOH three years ago and, though you still get together occasionally, you've grown apart. You've become much closer to your coworker who you spend every lunch hour with and most Saturday nights on double dates. Chances are, if you don't feel the same close connection to her, neither does she. Invite her to the wedding as a guest and don't look back.
She'd be thrilled.
If one of your possibilities is time-crunched with a new job or baby, for example, vote her off. You're not being mean, just practical and considerate. Choose someone who's available, physically and emotionally, and would be excited to take on the job.
She's seen you undressed.
This isn't mandatory but totally helpful when using the toilet while wearing a wedding gown. When you pee, your sense of modesty gets flushed down the toilet along with the Charmin, so it's less embarrassing if your helper, who's holding your dress aloft while you do your business, is comfortable seeing you with your (under) pants down.