Most mother-of-the-bride hairstyles are just a step up from your everyday look. You most definitely want to be careful that your style does not outshine the bride's. Generally the mother of the bride wears her hair down. Trying to be too formal or going so far as to wear a wedding updo will steal the spotlight from the bride on her special day.
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Get a professional cut and color (if applicable) a couple weeks before the event, so it will have time to settle back to "normal." Then, on the big day, you'll be ready to simply take your everyday look up a few notches. And plan to make a trip back to the salon on the big day for a professional 'do—you have enough to worry about without having to fuss about your hair!
You might want to go with a soft curl look, which can be achieved with a set of hot rollers or curling iron. Gently break up the curls with the fingers to soften the curl and add body. You might also want to add a fancy hair clip or barrette to keep the hair off the face while adding just a spark of formality to your style.