First rules worth setting as a keyholder?

Someone I have been talking to has asked me to be their keyholder, and I have said yes. I want to do this well rather than wing it.

For the experienced keyholders here, what rules did you set at the very start? I do not mean anything strict for the sake of it, more the basic structure that keeps things clear and safe for both people. What did you wish you had agreed on before you began?