I believe there are many aspects to building trust in a relationship. I do believe that respect is essential. If you lose respect for your partner, you will eventually lose the trust. Communication is important, but can be meaningless without complete honesty. If you have open, honest, meaningful communication, trust can grow and strengthen. Forgiveness is also very important. We all mess up sometimes. We say the wrong thing, do the wrong thing, or hurt our partners sometimes without even knowing it. We must be able to divulge and forgive. No holding grudges! In my experience, the grudge holding can start a chain reaction that can severely damage respect and trust between two people.
As for spanking, I do believe the trust must be there before you engage in this activity. I also believe it can strengthen the trust, love and respect you have for your partner and bring you closer together as a couple and as friends. I have only expirenced a small part of this lifestyle but I know it is something I need to explore more.