When choosing clothing for a portrait shoot, opt for solid colors. Rich jewel tones work especially well. Stay away from bright white (ESPECIALLY if the other person is wearing dark clothing), busy patterns (which draw attention away from your face), bulky clothes (which make you look bigger), and turtlenecks (which make you look shorter and, well, neck-less). Match your clothes to the location - trendier, edgier cloths for an urban, gritty location, or more classic clothing for a park, for example.
If you are posing with more than one person, make sure your clothing doesn't clash! A family or engagement shoot will look dynamic when everyone is dressed a similar color and style. Try laying out all the outfits beforehand to make sure they go together well and that no one piece stands out too much.
If we plan to be moving around a lot during your portrait session (i.e. seniors or engagement sessions) and you're going to wear killer heels...consider bringing a pair of flip flops or flats to wear while walking; you can change back into the heels for pictures. I put that in bold because I promise you - you'll be so glad you did!
For a more detailed list about what to wear (and what NOT to wear), check out this post by my friend and amazing photographer Stacy Reeves.