Keeping Sexual Passion Alive in Marriage

How does a man stay sexually interested in his wife after years and years of marriage? Jesus said that anyone who looks at a woman with lust has already committed adultery with her in his heart (Matthew 5:28), but this strikes me as being terribly unrealistic. In my experience, sexual attraction has always been closely tied to anonymity. A good-looking stranger excites my imagination precisely because I don't know her. My wife, on the other hand, is as familiar as an old pair of slippers. This seems only natural, but it really bothers me as a Christian who wants to stay faithful to his wife. Can you help?