It's important to choose your makeup and hairstyle AFTER you choose the dress. You want to make sure your hair and makeup complements the dress and vice versa. I always recommend trying on the dress and then playing with your hair in the mirror with a friend or family member helping you decide what would look best: An updo? Wearing hair down? A bouffant? A braid?
When it comes to makeup, a good rule of thumb to follow is to either play up the eyes or the mouth. Then take your cues from your dress. Would smokey eyes look best or should you go for a really bright pink lipstick?