When asked why I am opting out I don't say anything or just respond "really".

One tactic that I noticed during a recent opt out at JFK is that the TSA shutdown the whole line when I opted out. They made everyone else behind me wait a couple of minutes until a male assist came over. After a minute or so the TSA guy asked me if I still wanted to opt out - as if he was trying to pressure me into not opting out as comments were starting to come from the line as to what the hold up was.

If I do have a medical reason for opting out, such as when I had a fractured ankle and could not easily take off my shoes, and am asked to explain my medical condition I respond with the following:

ME: "Oh, I was not aware that you are a trained medical professional. Where did you attend medical school?"

TSA: "huh??"

ME: "lets continue to the pat down"