In addition to love and respect, the answers to this question are very simple -- yet hard to establish and maintain.
1. Complete, 100% honesty -- lies, no matter how insiduous, are deceptions that lead to other deceptions in a relationship. There are lies to hide, there are lies to save feelings. The second variety is particularly hard to break -- until you really think about how disrespectful this type of lie is -- it is essentially saying that your partner is too weak/immature to handle the truth and that you must protect them at the expense of your true feelings. Of course, tact and timing is key. Learn how to be positive in potentially negative situations.
2. Agreement on money. (Committed) partners should choose a dollar amount (it's $25 in my wife and me's relationship) where you can buy anything up to that point, but anything else has to be agreed upon by the other partner. And no penalizing if you don't get your way!
3. Partners need to know where eachother are at all times. There's no valid reason why you shouldn't be aware of your partner's schedule. Taking time to talk about eachother's days/weeks is a sign that you care about what the other is doing -- and is necessary in case of emergency.
4. In public/private situations without your partner, don't do anything that would hurt your partner's feelings/anything you wouldn't do in front of your partner.
5. Learn to argue productively; don't worry about who is right, worry about what works to solve the problem; don't throw (resolved) history in the partner's face.
6. Very important -- don't put yourself in situations where cheating can occur. That simple. Just about everybody, no matter how committed, will cheat under the right conditions. Don't let those conditions exist. Not spending enough time together? Sit down, talk about it and spend more time together. Mad about something? Resolve it. Etc., etc...
You have to decide that you want a relationship and work to protect it if you are to remain faithful in anything. Your partner is supposed to be the person closest to you in the world -- it is your responsibiltiy to protect them, their feelings and the relationship, even if it means some perceived personal sacrifice -- what you gain will be much greater than what you lost.