You should wear clothes that best suits your personality, you are comfortable in, and reflect the style of photographs you're interested in. In general, darker shades are more slimming, shirts with sleeves make the subject appear slimmer and solid colors work best as stripes, plaids, checks and bold prints draw the eye away from the face.
Keep your makeup simple unless you are going for a high fashion look. Bring some touch-up makeup along for use during the session. This includes powder for taking off highlights on the skin, and yes, this can be used for the guys too. Keep your jewelry simple as well. The focus is on you and not the jewelry. It should be subtle like small necklaces and earrings.
Have your hair done before your sitting. Bring a brush or comb to use during the session. If you would like to get some photos with your hair up then bring a hair clip or band. Have your nails done since they may be visible in the photos. A fresh shave is a must. If you have a beard or mustache, trim it to your liking.
Do not worry if you develop a blemish. The finished product can be retouched to eliminate blemishes and soften harsh lines.
Express yourself by bringing items to your session such as a musical instrument, athletic uniforms, sports gear, pets or anything else that means something to you and makes your portrait unique. Think of the kind of poses you would like to do. Clip out poses from magazines or print from the web and bring them with you.