This has happened to me a couple of times. When the provider tells you that you are 1) fun to be with, 2) easy to talk to or 3) a cool guy they would like to see again, that is usually the warning sign that things are about to go in a direction I do not want it to go in. At least in my situation. So I am very clear with them on my expectations and the time period I have available. I usually make them aware that I have plans for later (dinner, drinks with friends, going to a movie, work etc). I have also used my phone as a crutch by setting an alarm so it will alert us both that time is up. Don't try to be subtle - it doesn't work. Be respectful but direct; that is the advice I would give.
By the way, I always avoid alcohol - it is one of those things that makes you slip...