For H.S. Senior Sessions, my seniors usually like to have their photos taken in two different outfits. I always suggest one casual and one a little more formal.
Try to choose your colors so both outfits coordinate. This always looks so much better when framing your photos next to one another, creating a photo postcard with multiple images or sharing your photo gallery (which is a beautiful online collage of all your photos).
Here is an example of a few outfits I put together…see how well they coordinate.