I post on the appropriate board here on ScubaBoard.com saying where I'm going and when.
First of all, I agree with this. I have done it several times and always found success.
On the other hand, I feel your pain, because I have been there often and had some bad experiences. It is because of that pain that I now usually go the SB route.
When I have not done that, I have to feel people out, as Jim said, but they are feeling me out as well, and they may be reluctant to take on someone they don't know. Sometimes you really don't have many choices. In fact, I have been on dives where I essentially had no choice at all. I have been on more than a couple of dives where I had to return with tremendous amounts of unused gas because of the buddies I was with.
That is why I now prefer not to get myself into that situation.
One thing I do is the opposite of what many other people advocate. When I sign up, I always show an advanced level of a certification card, usually an instructor card, and in my experience the operator goes out of the way to get me a good dive. I know that is the opposite of what some people say, but that has been my experience.