A very important starting point overlooked by almost everyone is the environment that you will be getting dressed in and going out in. Investigate the outing — show, shopping, theatre; and what will be the lighting, temperature, dress code. Lighting is really critical — try to match the expected lighting with your makeup room (mirror) if at all possible. Fluorescent cool-whites are a very different colour from light bulbs (tungsten) and totally different from the sun. Hence a room with a window and the three light types approach the optimum situation. Your choice of foundation is really governed by the lighting in a store, for instance.
Clothing is another matter — talk to your associates about this!