Here are few key things that I've learned ...

First, clear communication is very helpful.  Really, its necessary for any relationship in any context but it can be taken for granted with those we know really well; so you have to clearly articulate what helps, what hurts, and what you prefer.  The tricky part here is to do it in a detached way, instead of a reactionary way, because then the other person will be more open to receiving it.

Second, forgiveness is important.  The flip side of this coin is to see each feeling of hurt as a chance to cultivate your love.  At some later stage, we may get to a point where, like Gandhi, someone could put a bullet through us and we bless them -- but to get there, our intimate relationships are like the swimming pool where we can practice our skill.  Somewhere I heard, "In the West, you marry the person you like. In the East, you like the person you marry."  There's a lot of merit in understanding nuances of that as well.  If you feel hurt, whether you're justified or not, practice looking at yourself and rebounding that feeling with greater love.  And if both people do this, as a common protocol, it can yield some smiles of real beauty. :)

Third, mutual commitment to a deepening awareness really helps.  Very practically, its a waste of precious time together if you create problems and then solve it; in fact, some relationships get so obsessed with it that they feel almost lost without problems.  You don't want to create that pattern.  So look at this deeply and ask each other the question of: what do we really want out of life?  Let that be your organizing principle.  And how can we support each other in living that ideal?  And if you don't yet know what you want from life, then that itself could be your starting point.  But key point here is to build your awareness to a point where the little things really do stay little.  Don't get self-obsessed, don't support litte ego-trips (which can turn into big trips), accept each other as you are and help each other live out your highest ideals.

Fourth is service.  Ultimately, you have to be in the relationship to serve the other person -- if you are going to be in relationship, you have to be so madly in love that you care about the well being of the other person so much that it takes precedence over yours.  If you truly head in this direction, you will constantly ask yourself: what can I do to support my partner in being the best person he/she can be?  And if you truly are in love, you will notice that this person was simply a trigger to ignite a love within you that can shared with anyone and everyone.  That realization, that experience, births an incredible gratitude.  And from that place, you will only feel like serving the other person even more.  You will make all the sacrifices and yet feel like the king.  Everything becomes an excuse to say thank-you, and the more you give, the more your heart opens and we want to say thank-you even more.  The whole cycle changes.  You start to find yourself.  And when two people are in that space in their relationship, that, in and of itself, is of service to the world.