Validation is key. People need to validate each other constantly, by acknowledging each other, in their roles, in some way, complimenting, reacting to a move. The successful relationships seem to do that. Attitude keeps it hot. Renewal is key.
Communication is important, love, trust, respect.
The slave's commitment to his role is what makes it work. A Master commented that he likes surprises that honor a slave's committment. Hourly reminders of that relationship can be helpful. A scene is not the same as a relationship. Some said slaves don't want lovers, that's the conflict.