A healthy premarital relationship is composed of two people who WANT each other, but know that they do not HAVE each other.
If a person HAS another, then they take the other for granted. If they WANT the other, then they work at the relationship. If only one wants and the other has, then the relationship is unbalanced. Work at wanting and never think that you have someone. Because the moment you believe you have them, that's the moment you won't anymore. And then you'll want them all the more.
This is also applicable to pre-marital sex. The foundation of your life-long relationship should be built on desire, not lust. It needs to be built on WANTING the other not HAVING the other.
Of course once you exchange vows, it all changes. But you still must be careful to keep from taking each other for granted.