Not to brag, but my relationship is perfect, and I'll tell you my little secrets for making that happen. Pay close attention to Rule #3.

Honesty is ALWAYS the best policy.
Even if it's something the other person might not necessarily want to hear. I ran into an ex at a bar last November and ended up talking to him till 3 am. I went home and immediately told Nick. He obviously would have preferred that I didn't talk to an old flame until the wee hours of the morning, but I didn't do anything wrong and I was honest about it, so how upset could he be about it?
I used to be friends with this guy who had a jealous girlfriend, and he would keep our friendship a secret from her because if she knew, "it would just cause problems". Then she found out anyways, and it looked like there was actually something to be jealous about because he was so sketchy about the whole thing. Honesty is always the best policy.

Don't change in front of each other.
I had a friend who told me that she didn't even notice when her boyfriend got naked in front of her, because they had lived together for a while and he undressed in front of her all the time. She was 21 YEARS OLD at the time. That was really sad to me, so I swore that I would never change in front of Nick. But after living together for a while, it is tough to hide every time you get sick of having pants on. So I sort of compromised. Now what I do is, if I'm changing and he looks at me, I act like "oh my god don't look!" and try to cover up the body parts he should not be looking at. Obviously nothing he hasn't seen before, but it keeps the mystery alive, you know?

Pick the right person!
Too often, I see my friends start dating guys and then turn into shitty people. But then on the other hand, there are people like my friend Krysta. Krysta used to be sort of shy and introverted, but then she met Clayton about five or six years ago and she totally transformed. She is now more open and ok with herself than I ever would have imagined she would be (and she's married and has a kid). And honestly, Clayton didn't DO anything special to change her, he was just the right one for her.
Same goes for Nicky... he's just the right guy for me. And no matter how many times I hack into his computer and play awesome April Fools Day pranks on him, he isn't going anywhere.
The best thing you can do to maintain a healthy relationship is be in one with the right person.