Master T, my hubby, and I don't have a 24/7 BDSM relationship.
We're best friends, lovers, parents, etc. and play Master and Pet/Sub as often as we can. We have very different work hours. So scheduling and timing are important.

Sometimes, I just have to say - "I want you, tonight" and that's enough to rev his engine and get me in trouble if I leave him walking out the door with a woody.
(Naughty me.)
Othertimes, he responds with, "How do you want to play?"
That's so tough for me, cause we are so busy trying to please each other, that I'm not sure if I'm supposed to tell him what he wants to hear or what I really want.
(I'm not one for mind games. Just tell me what you want and I'll do my best to give it to you, but make me guess what you want and I get very, very cranky.)
I've learned to answer with what I want and let him make the decision as to whether or not I receive it.

Short answer: It works best for us to let each other know what we want and to set expectations ahead of time.

That's not to say we don't enjoy immediate gratification or surprising each other, but we have to be very careful of "teenager-interuptus".