Bend deeply at the waist before fastening your bra—this automatically centers it and distributes the volume evenly. Straps should neither fall down nor dig into your shoulders (they ought to provide only 10 percent of your support—the rest comes from the band).
Buy a size that fits you very closely—it may even feel a bit tight at first—when fastened on the first (loosest) hook. This will enable you to tighten the bra over time as the material stretches out. Make sure that the band is perfectly horizontal on your body.
Fix minor puckering in the cups by smoothing down and "tucking in" your breasts. Fit check: If there's a major gap between bra and body, the cup's too big; if you're overflowing the top or sides, it's too shallow. Wacoal bra, $48.