Once you've been with someone a while, you have to find a balance between the comfort of ritual, of your body knowing what is about to happen, with the unexpectedness of new things and pushing boundaries.
The action in most romance books happens at the beginning of a relationship, when everything is still new. In a power exchange relationship you can often see the Dom laying the foundations for a particular ritual even in the first couple of scenes-- usually in the way a scene starts. That's helpful, as it helps the submissive begin to get into the right headspace. If you are only writing a few scenes for this couple then that's fine, but for a lot of scenes it would present more of a challenge.
In real life I prefer ritual at the beginning and end of scenes with the stuff in the middle an unknown. Sometimes I get a ritual that lets me get ready in my head before the scene starts, sometimes I don't. In my writing it depends on the characters, I think. A more hard core sadist working with an experienced masochist quite possibly won't always want to give the starting ritual just to keep the submissive that much more on edge for a scene. But in a relationship with less S&M in it, or one with a less experienced submissive, that ritual at the beginning can be pretty important.
Sometimes a ritual can be as simple as kneeling or being bent over something while the Dom collects the tools that will be needed for the scene. Other rituals can take much longer - the Dom who gives the submissive an enema in preparation for a fisting scene, for instance.
In that particular scenario - let's say the Dom has a huge ritual around the enema that includes a small enema, then a large soapy enema, then a rinsing enema. This is followed by a sports drink and a nap, then spanking or flogging to subspace while plugged, and finally being bound a particular way before the fisting portion of the evening begins. I could write a wonderful scene around that. However, if the Dom sticks to that exact formula every time he wants to fist her, at what point does it leave the comfortable ritual feeling and become the same old thing. Even if it only happens three or four times a year, I think that a few years down the road it will lose most of the excitement factor. The key, I think, would come in finding ways to mix things up. Cold water versus really warm water enemas, possibly skipping the nap without warning, and certainly in finding different and creative ways to tie her up. And perhaps most importantly, doing things other than fisting after the ritual, so she's not sure what will happen later, only that it will likely involve some kind of ass play... an anal speculum perhaps, or a figging scene.
I've discovered that the world of fantasy doesn't always relate to the real world as closely as one might think. My stories so far haven't included much ritual, because too much ritual in books equals a pacing problem. So far I've only included ritual when I've skipped forward a bit in time and I show how comforting it can be to go through an already established ritual.
How important is ritual to you in real life? At what point does a ritual become the "same old thing"?
How about books? Is it important in books? Do you appreciate it? Or would you prefer that each scene be fresh and new from beginning to end?