There really aren't any "tips" that I can give you. But my best advice would be...don't hold back. Obviously that doesn't mean you have to be mean/cruel about what you say. But say how you feel, how her behavior affects you...state what it is you need, what you want from your M...where you want it to go etc.
Since this C is willing to see both of you I might ask if he/she would be willing to see you for a couple of sessions on your own as well though. I mean...he/she has been seeing your wife one-on-one for a bit, this C needs to get to know you a bit too....and I think it's best done without the tension of your W sitting right there. That way you can feel free to say ANYTHING you need to say. Then, your C will be able to also ferrett out some of the real truth....and help guide both of you when you are in joint sessions.
