1. Trust is one of the big factor to last a relationship. If one don't have trust in his/her partner then the relationship is in a dilemma.
To establish trust they should have an open-communication. No secrecy or no hidden information so that they would know how deep is your trust to each other.
2. Respect is also a factor. If one says No then the partner should respect it. Relationship is a give and take relationship. Respect is sometimes missing in a relationship.
To maintain respect they should first establish or know the likes and dislikes of their partner. Communication plays also an important role in making respect. Argument is a sign of lack of communication and respect to a relationship.
3. God must be in the center of every relationship for it to last. If God is not in a relationship then it will be useless.
To maintain a Christ-centered relationship the couple should share their own religious views and perspective. Continuous prayer is needed to establish a Christ-centered relationship. With God in the relationship I'm sure it will last forever.