The best way to keep your girlfriend happy (besides the obvious things like honesty and faithfulness) is to make sure that she knows that you love her. Leave her notes where you know she will find them. Cook dinner for her (or take her out if you can't cook). Buy her small, thoughtful gifts. (If she's the sort who is only impressed by large, expensive gifts ... then maybe it is time to find a new girlfriend.) And most of all, make time for her and be interested in who she is and what she cares about.