Sarah is a pretty common name. I'd avoid the "standard" middle names (i.e., Sarah Elizabeth, Sarah Anne, Sarah Marie)" because she's bound to run into a ton of other sarahs in life with these middle names. How about:
Sarah Camille, Sarah Lisette, Sarah Natalya, Sarah Fleur, Sarah Corinne?
Maybe your maiden name, or your mother's maiden name? Grandmother's first names? The name of a favorite character from a book or movie, or someone you admire? I always like middle names that have some personal meaning, rather than being chosen just because they sound nice.