I hear what you are saying---sometimes for us it's been because someone else is in the session (switching)--but we've worked that out....

the other thing that my T and I did was to set a consistant day and time--same two days every week (except for vacations and such) and the same time. This has helped with the consistancy in that regard. before we did that, it was confusing and we forgot appts and were feeling like there was no set anything.

We also set an agenda of sorts for the next session--for example if we are working on a specific thing, she gives me an "assignment" for the next session and then she checks to make sure nothing significant has happened in the last couple days (between sessions) then if not, we continue what we began the last session. It has really helped us to feel like we are moving forward.

times when she is away she gives us something to "work on" while she is gone and then we pick it up when she gets back (unless something has happened that needs to be addressed first).

it has worked for us.
maybe it will work for you too.....