Great question that is not asked enough!

To me a good start into a session is as important as a good foreplay in vanilla...

During the whole session I like to be under physical control. So for me a session starts once I am tied up or chained down. Hence the process of restraining me doesn't really count as session yet for me - at least mentally. Something I always mention in my pre-session briefing.
So if she starts to command me around or inflicts pain before I am restrained, it makes for a bad start and is often a sign that she didn't pay attention to the pre-session conversation.

About the end:
The longer it takes me to get back my "every day mind" the better :-)