Basic Everyday Panties
Comfort is the first priority when selecting a panty. Look for soft fabrics, smooth elastics at the waist and legs. Panty should not be tight, particularly at the waist and legs, but elastic should hold the garment in place. For a smooth look under clothes try styles that do not have too much extra fabric or trims. Your bottom back should be gently cupped and the crotch should sit naturally against your body. If you feel uncomfortable, try the next size.
Special Occasion Panties
While comfort is still essential these come in a variety of styles and fabrics, and the fit will be different. These will often have a sexier fit - high cut legs, low rise skimpier back, and often have more embellishments - lace, ruffles, bows, etc.
Caring For Your Undies
Always follow manufacturers care instructions to obtain optimal wear from your undies. Periodically check to ensure that the fabric and elastics are performing acceptably. The fit of those that are worn most often will probably start changing - feeling looser on body, riding up. Then you know it's time to replace them.