I have recycled some of my older, striped English shirts as pocket squares. I have taken an 18-20" piece of fabric from the back of the shirt, hand rolled and basted the edges, and woila, a great cotton pocket square. And a reminder of a former sartorial friend.
What a capital idea. (Sorry, Mr. K.) I love it. Not that I would want to purposely cut short the life of a decent shirt.... And I personally use both silk and linen squares, only on occasion, and I generally follow Manton's wet/dry recommendation, though I had never heard it described as such. Sans tie, I prefer to go silk with the pocket square.