If familiarity breeds contempt, then please give me some ideas on how to keep my husband "on his toes"?

Let me start by saying I have been married for four years, I have not let myself go, I am more attractive than before we got married. I am fun and adventurous in bed. I make sure I give my husband attention, love, and romance. I don't need tips on how to spice it up, I need help in knowing how to break habituation. I've caught him checking out other girls on the internet and getting little crushes on girls at work. these girls do not have anything on me(looks or personality) except they're DIFFERENT. he hides it well, but I really need to get him realize what he putting in jeopardy before he loses it all, me and his family. I love him, but I can't take this to my heart or my ego. How do I stimulate that part of him that will make him open his eyes and see how lucky he really is and how much he has to lose.