For the ladies, If you plan on cutting your hair before you photo shoot or portrait session, I suggest you do so 1-2 weeks in advance. The reason being, if something goes wrong, you have time to correct it, especially if it's a whole new look. For guys, a few days in advance is fine, but the same rule applies if you're going for a new look.
Ladies, bring your favorite hair accessories, for different looks. If you're shooting outside, you'll be glad you brought them.