Certainly this question is open to debate, as there is no formula that would guarantee a relationship to always be in great terms. However, one might want to consider what the experts see in BAD relationships in order for us to filter and avoid bad choices that may ruin relationships. This applies to both casual, personal, professional and family relationships.
The first denominator for a healthy relationship is, undeniably, open communication. If you notice, most seminars in qualitative customer service, dating services, church counseling services, and even therapy, consider that communication on both parts must be open and impartial. This means that the parts involved are able to speak their minds, state their wants and needs, and share their opinions without the fear of being censured, criticized, or judged. Conversely, a bad relationship is always characterized by the lack of speaking and listening, which in turn, leads to confusion and assumptions that should not exist.
The second denominator is mutual support. In any award winning organization the members and sub-groups share a common goal. If you are expecting someone else to move you toward a goal, you are literally fried. It is the mutual want to achieve a common achievement what keeps the synergism alive between the two parties. Imagine a marriage where the husband wants to spend his fun days with his wife, and a wife that wants to spend her fun days with her friends. That is not mutual support. That is not communicating to express the real needs of the couple.
Finally, one last important factor is the ability to make commitments. This has nothing to do with engagements, marriages, nor promise rings, but commitments that show that both sides of the relationship can go above and beyond the expectations to help each other succeed. We do this with our children most of the time- when we take time to help them instead of doing what we want to do, when we shut down our TV to listen to their drama...even when we go into debt in order to provide them with a good education.
Those are just some factors that play a huge influence in the process of developing and sustaining a healthy relationship.