Only the top half of your body is visible in headshots so pay most attention to what will be featured in this section of the image and bring more choices for the top half. Don't worry too much about shoes as we probably won't be seeing them in any shots (unless we decide to do a full length shot as well on the day). If wearing a jacket, keep it fitted and think of a top/shirt underneath that will add a pop of colour. Primary colours (blue/yellow/red) work well as do jewel colours jades/pinks/silver/gold).
Avoid clothing with busy patterns. Go for solid colours. Avoid plain black or white (where possible) as they can wash out skin tones. Large logos are a no-no as they are too distracting and don't date particularly well. Form fitted and tailored clothing is best.
Layers are good too so that you can mix it up quickly to give a totally different look.