Be Warned: There now follows a long-winded explanation of a small tip that's probably of only marginal interest to a very small number of people.
This is my sturdy Oyster card. It enables me to travel around the sprawling metropolis and win at Chromaroma.
I keep it in my wallet.
This is a card I was recently given to enable me to access a building where I occasionally work. I keep this in my wallet too.
But then I discovered that the access card interferes with my Oyster card (though not the other way round). I presume it has an RFID chip in it. It meant I had to remove one or other from my wallet every time I went on through the gates to the tube, which was a tiny inconvenience.
(Subsequent experimentation confirmed the presence of RFID)
So I spent time googling for a wallet with a built in Faraday cage. Unfortunately they were either ugly or more aimed at those who were worried about RIFD sniffing - total blockage rather than card separation. But, in reading this I realised that all I probably needed was a thin strip of metal to go in there somewhere.
And it further occurred to me that I already owned thin strip of metal that I used to carry around in my wallet but removed because I was unlikely to need to prove my Long Now membership status in an emergency.
So I put it back in, next to the access card, with the oyster card and the access card separated by everything else in the wallet and hey bingo it works fine. So, er, if you're a Long Now member and you have an oyster card and something else with RFID in your wallet that might be worth knowing.
For everyone else - I bet this is going to be more a common problem. Keep your eye out for long thin bits of metal.