Some women revel in the thrill of the chase. Unfortunately, after an extended period in a relationship, that thrill can fade. You can remain a challenge to your girlfriend long into the relationship, though. You need to make a few adjustments so your girlfriend feels like she still has to work for your attention. It requires a lot of tact and finesse to stay right on the line between challenging and neglectful, though. If your girlfriend starts to feel neglected because of your aloof attitude, pour on a little romance to make her feel appreciated.
Maintain a healthy level of distance between you and your girlfriend. This means you should enjoy your favorite hobbies that don't necessarily involve your girlfriend. This doesn't mean you will blow her off to go pursue your interests, but it should encourage her to chase after you and participate in your hobbies.
Limit the amount of cheesy affection you give your girlfriend every day. There's nothing wrong with initiating a cheesy "No, I love you more" contest occasionally, but you shouldn't always be the first to initiate romance. Handing out your affection freely (for example, saying "I love you" numerous times a day) can make your girlfriend take it for granted.
Distance yourself from your girlfriend when she treats you poorly. Show her that if she wants you around, she had better play nice. Give her a cool down period (try two or three days) before you contact her again to see if she is ready to be nice.
Stand your ground when your girlfriend makes unfair requests that put strain on your free time. You need to show her that you won't let her push you around and control you. For example, if you made plans to spend time with friends, don't cancel on them because she doesn't appreciate your friends.