Often clothing style is dictated to some degree by the type of portrait : business or corporate, formal or relaxed, indoors or outdoors. It's best to have a few outfits on hand to try on during the first sitting as one may work better in a certain setting or light than another. A good rule of thumb is to feel as comfortable as possible in your clothing, so that you can feel relaxed during the sittings.

As most of the attention will be around your face, think of the neckline and jewelry for women, tie or collar style for men. Avoid trendy clothing unless it suits the type of portrait being painted. Likewise it's preferable to have no--or minimal--makeup to avoid distorting skin tones.

An advantage to a painted portrait is that unlike photography, the goal is to achieve an excellent likeness but not necessarily reproduce every detail! So minor issues such as clothing wrinkled from travel are not a worry.