Getting somewhere

I think I know what people mean by don't force it. Finally got the chance at a session by myself last night, and I noticed that your body really does the work for me when I clench my anus for a while. Once it started fluttering and trying to relax that helix was really moving quite nicely over my prostate. How do you keep it going like that, when the stronger sensations start to build, though? It's hard not to let go, but once I did I had to start all over again from square one.
Also, I have seen a couple people saying to know when a session is over, I'm guessing this was when my anus got so tired that I couldn't control my contractions anymore. Any insight?