We recommend a simple approach. The classier clothing you choose, the more timeless your portrait will be. The following are two general ideas to consider when choosing clothing (especially shirts/tops). These ideas will drastically improve the quality and look of your portrait.
Solid colors (even with texture) ALWAYS look best. Contrasty patterns will make a portrait about the clothing instead of the person in it (floral, stripes, polka dots). Be careful with the use of patterns in clothing.