Two people in a healthy relationship experience less stress and are more capable of handling a stressful situation when one comes along. Know why? Because they have help, that's why. They have someone who supports them in every way. They zig, the other zags. It is just that simple.
The kinds of things that make a good relationship are effective communication, trust, mutual respect, being flexible when it counts and keeping everything real. If you have these things in your relationship then you probably treat each other as equals and do not experience too many problems.
By keeping everything real I mean that you should not expect your spouse to do anything you would not do. Being in a relationship is a two way street so do not expect more from your spouse than you are willing to put forth yourself.
I also cannot stress how important communication is in any relationship. This is the basis for keeping things healthy. If you talk about your feelings then there can be no surprises and no one will ever get blindsided. If something comes up that may be a little difficult to talk about find the courage to talk about it. Otherwise it will just fester and soon the good relationship won't be so good anymore.
Part of good communication skills is the ability to listen. Have the courtesy to remain silent when your spouse is talking, do not interrupt and do not try to fix it right away. If a fix is needed then figure it out together. Remain partners at all times.
Disagreements happen. No one is perfect. Handling them with dignity and grace is important. Do not attack your spouse but do not be a doormat either. If you have done something wrong apologize and then work together to learn how to get past it. Having the ability to forgive and forget also is key to keeping a relationship healthy. Do not, under any circumstances throw things back in each others faces. If you gave forgiveness once then let it go and move on.
Show love and genuine concern for your spouse. Be there for each other to lean on. I have found that a healthy relationship does not mean that each of you is strong at all times. When one of you isn't then the other one must be and vise versa. This is what committing yourself to another person is all about.
Obviously being faithful is also part of having a good relationship. If you have committed yourself to someone have enough respect for yourself and them to remain faithful and do not go pandering about with someone else. This is the worst thing you could do in any relationship and one of the biggest reasons why relationships fail. These things are all what a healthy relationship is all about.