There is an excellent book about Boundaries by Dr. Henry Cloud and Dr. John Townsend.
I've heard them speak and they offer an excellent view of how to put boundaries in place. I'm currently reading another book by them: It's Not My Fault, which is also recovery based....all excellent helpful material.
Boundaries are just a way to protect you and to let others know that you respect yourself.
I use a method similar to what Mike does... I 'take my situation' and imagine a friend in it and that helps me to 'see' if I am allowing behavior that I shouldn't.
A boundary is a fence or wall...and if I don't have it in place anyone is free to do anything they wish. I can place the boundary on myself....by leaving the room if need be or by refusing to join into an argument. I can change the boundary too...because it's there for me! I'm in control of it.
The good thing about boundaries is... once there, I have less negativity around me and sometimes...sometimes, people begin to change how they treat me, which is a nice added benefit but not the motive to have a boundary.