Kick it all up a notch. Your portraits are you on your best day, so make it that best day. Get up early, spend extra time in front of the mirror, make everything perfect so that when you get to the studio your ready to go. If you need any last minute prep time, arrive early (not late). Think of everything when it comes to accessories; from your hair and makeup to your socks and toenail polish. You never know, you might be photographed barefoot.
Eyes-Well groomed eyebrows are a must (both guys and girls). Frida Kahlo is the only person I have seen pull off the bushy brow look with style. Enhance your eyelashes with a "little" black, waterproof mascara. Dark brown eyeliner is a good alternative to a heavy black eyeliner and compliments all shimmery eyeshadows.
Face-If subtlety is your style then keep it to a minimum, but don't skip foundation all together. A little bit goes a long ways in photography, and just the opposite, a lot goes a little ways to improve your appearance. Don't cake it on, our images are all fully retouched, so let us take care of that blemish for you, you just worry about being yourself. Pay close attention to base colors so that you don't discolor your face in relation to your neck and arms. Use both SPF-free moisturizer and foundation to prevent reflection and to enhance your skin's natural glow. Use a translucent powder to set.
Cheeks-Don't be afraid to have a little color on the apples of the cheekbones (petal pink). It really brightens up the appearance of your face and gives it life.
Lips-Last but never the least. Moisturize, moisturize, moisturize. Work with great glosses full of color and shine. Berry colors, deep reds & browns will make your lips gorgeous. Avoid frosty pinks and white tints for portraiture.
For the men
You don't have to cut your hair but you do need to comb it. If you are going to cut your hair, do so a month earlier so that you can get used to it and have a chance to grow it out a little if someone gets a little too robust with the shears. If you do not have a full beard, don't bring a half beard. Be freshly shaven the night before so that you don't have razor burn the day of your session. Check your full clothing choices with your parents a couple days before so that everyone can feel good about what you will be wearing for this once-in-a-lifetime event. (If your really smooth, you could show them some really bad choices and say you have nothing to wear, then off to the mall you go to get some great new clothes to wear for your photo session and your next date.)