First, think about what kind of portrait it is. Is it for a business headshot, family portrait, etc.? Each one requires something different from casual, semi-causal, to full dress in suit and tie, or dress. But, here are some basics to think about for your next portrait:
1) Wear clothes that complement your hair and skin color. You know what looks best on you, but if you don't know, get a trusted opinion from a friend or someone in a clothing store.
2) Stay away from stripes and logos. Graphic items like those are distracting to the viewer of your image. You want them to remember your face, not your loud clothes.
3) Don't be too revealing. I say that because you want them to remember your face, and not other parts of your body. This is very important for business portraits.
4) Make sure your clothes are pressed and ready to go.
5) Don't put on more makeup than you normally do. Try to keep it light and natural, wear what you wear on a daily basis, or if you like, put on what you would for a casual night out.
6) If you are going to get your hair cut, do it at least one to two weeks prior to your portrait. This will give your hair time to settle and look natural.
7) Get good sleep the night before and drink plenty of water. Why water? Water will hydrate your skin and make it look healthy. Also, it will reduce the inflammation around the eyes. So, don't drink any alcohol or caffeinated beverage the day before, or the day of your portrait.