Take her out somewhere special... somewhere fresh besides a movie or diner... But every thing everyone else said can work with this to! like (@TaylorF) telling her how much she means to you! or (@alexistheawesome) getting her something.. just don't get her something to expensive depending on how she is she might feel like she owes you for it!
Give her something that she likes and never could get.... take her somewhere and than tell her how much she means to you... or just talk about is disappointing her and let her get it all out because that also shows how much you care about her and her feelings, but if she is disappointed in you than tell her sorry and give her the gift or something.:)'