When they did that in Boston, the National Lawyer's Guild handed out big red buttons that said I DO NOT CONSENT TO A SEARCH. I put one on my purse. Just carrying it around, even if you don't use the subways much, at least shows other people who might not have thought of it that they do have a choice. They used to be available to download and make your own button, but it looks like since Boston isn't doing it anymore, they aren't on the NLG website. Still, it should be easy enough to make your own.

Of course it was completely pointless, because they only did it at a few of the larger subway stations, and all you had to do was walk a few blocks to go to another one.