First of all, a pair should have some common interests and hobbies. I mean that they always need something to discuss, so that time spent together will become much more exciting for both 'parties'.
Secondly, the main ground for healthy relations should be mutual respect. When partners respect point of view of each other, they won't have quarrels at all.
Moreover, they should always stay in touch. Some kind of pauses or so-called brakes can substantially damage the relationship saved.
Then, a couple shouldn't be stuck up in routine, since it always breaks down all the cheerful mood whenever you get it. There shouldn't be strictly defined rules of how to lead the household.
On the other hand, everybody needs to have some own time. I guess that here friends should take the stage. Some evenings can be spent separately in different circles.
A great helper for good relations is common tastes. It's really nice if you can discuss some favorite books and watch film which both of you like.
All in all, it's up to people how to preserve their relations. My husband and I have very similar tastes and opinions, but different characters, it adds some kind of spicy difference, since nothing can be absolutely perfect.