You've been dating him for a while, and your almost settling in as a true couple. Don't settle in just yet...here's few things to keep you two more than just another couple. He'll love you for it... for a long time to come.
Hold on to your own personal life: Hopefully you had your own life before you met him, so you'd better keep it when you have him. When couples spend large amounts of time together, they create a life together that consists of things you do together, share and will do in the future, but it's important for your own personal growth that you maintain your own interests and activities. Your interests are what make you unique, and keep you from becoming a stagnant entity, which is what attracted him to you in the first place. He will love the fact that you are an independent person who will keep on challenging herself, and always have engaging interests. Make time for your friends as you did before you met him, make time for yourself, don't drag him to things he's really not interesting in, like shoe shopping.
Keep flirting: Flirting isn't just for the newly introduced and recently started dating. Flirting with your long term boyfriend reminds him that you are still madly attracted to him, and that you will be in the future. It is important to his ego and masculinity when he knows you find him sexy. It makes for a better sex life, which is another important aspect of long lasting couples. Remind him of the woman he fell head over heels for by giving him coy compliments when he calls you from his job. Over dress when he least expects (hint: where something flirty in his favorite color.) Drop sassy compliments any time, "hey sexy" is a nice start, or complimenting him on his style or appearance while wearing a sultry grin works just fine!
Mind those manners: When you first meet him you're on your best behavior and slowly without your even noticing, your manners tend to disintegrated until somewhere down the line when you're with him you act as if you aren't in front of company at all. Slipping into a too relaxed state is common, but remember that his continued company is worth minding yourself. Keep yourself aware as the relationship grows. Things like belching out loud, ignoring him when he comes home or when you are eating together, running out of patience quickly. Remember, he's not your family until he puts the ring on your finger, don't treat him like he's always going to be around just because he always is.
On that same note, what are you doing with those rollers in your hair or the equivalent? Not going to the gym as much, forgetting the Listerine you used to use religiously? Fingernails just plain ragged? Well the girl who flirts with him at his work didn't forget these details. Basically it's just not a good idea to let your appearance decline just because you've already got him. Taking good care of your health and looks will keep your appearance and esteem at a healthy level that you'll both enjoy.
Boost his ego: He's got a big one, and he'd like to keep it that way. When he first meets a woman, and he gets those silly feelings in his stomach that he can't quite explain, and then he gets her attention and eventually her company it puts his ego on a high he wants to keep forever - let him have his way. This is one the most important things you can do to inspire him and keep him happy. His ego is tied to his dreams and his goals, so just dropping the odd compliment is not always good enough - anyone can do that. The best way to boost his ego is to show him that you believe in him and that let him talk openly about his goals in life and let him know that you believe that he can and will accomplish them.
Keep criticism at a constructive level, and temper it with a fair tone. He does expect, as well as respect, a fair amount of criticism, but be careful of your timing and tone. Critiquing him too much wears away at his temper and self esteem, but a healthy dose of criticism reminds him that you care, that there's room for improvement, and that he won't be getting away with anything. But the next time he leaves the seat up, or the toothpaste cap off, maybe you could let it slide. Save your criticism for something a little more meaningful. He forgot an important date? Let him know how it made you feel. He really messed up, you warned him, and now he's faced with the consequences, then you'll want to wait until the pinnacle of his anger or hurt is over, to gently suggest that he can avoid the problem next time by "blank".
These 5 relationship anchors will keep him close to you, and make him appreciate you more everyday that he's with you. He's worth the time and the effort, and by following these principles you can keep him in love with you for years to come.