If possible, hair is always done and dyed or close to. I have a bath and ex foliate and shave 2 days before to avoid razor bumps or anything, paint my nails or groom and polish them, etc. I pluck my eyebrows. I do a head to toe grooming!
Here's why:
Roots are distracting, chipped nail polish is distracting, etc. I want all of the attention in a photograph to go to what it's meant to. Whether that's an item of clothing, accessory, or just myself. It's important that the main focus of the photograph isn't a distracting "flaw". Yes, photoshop does a lot. But it is your job as the model to prepare yourself and do your part.
I pack my wardrobe the night before if it isn't being provided. And then I unpack and PACK AGAIN.
Here's why:
Options are AWESOME! And any photographer will tell you to bring 2-3 choices. And that's what they MEAN. But we LOVE our clothes, and we look good in so many of them (and out of them, right?) ! It's so hard to narrow it down. So the first time, pack all the things you WANT. Then unpack it. I find that when I go through my wardrobe choices a second time, there are ALWAYS items I end up removing. It's easier for you, and easier for the photographer!
And then I practice, practice, PRACTICE!
You know how professional models on television tell you to practice in the mirror? Omg do they ever mean it. It may seem vain, my friends, but I practice "good fayce" every time I walk past a mirror. Seriously.
Think of different emotions or states of mind. Curious, Sad, Angry, Sexy, Sultry, Happy, Innocent, etc. All of these have different expressions. The tricky part is getting the emotion or feeling or thought to be translated only by your body language and eyes!
Look at pictures of other models, poses, expressions, get ideas and be inspired. Then practice as much as you can! Learn how to find the light, and use it to angle your body and face.
Stupid arm. 
Nice Sundies