Men: A fresh shave is a must, unless you have a beard. If you have a beard, then it should be neat according to your own specifications. Whether you are doing formal or informal portraits, a long sleeve dress shirt or sport shirt, with a straight collar that fits at the neck, is a must. A sport coat or suit that fits at the shoulders, back of neck, and buttons properly at the chest and waist will help the appearance of your portrait. The tie is your choice and may over take your face if it is too loud.
Women: Make up should be applied for daylight. Eyeliner under the eyes is NOT recommended. However, mascara for eye enhancement is OK as long as it is applied correctly. For business attire, a solid color blouse or business jacket is acceptable with jewelry that is simple and doesn't override your face. The collar of your blouse will give you a longer neck if it is "V" or scoop shaped. Hair style should be something you're familiar with and not a change of look for your business portrait.
Remember, we do retouch so SMILE!