We only use cash in taxis after dealing with this annually for 4 years in a row (1/15 taxi rides would have the problem, but never on the way to the airport with our driver of choice - we call him directly - only when we we were taking the next driver in line.) The taxi from airport to house is $80 so it isn't a small amount of cash, either but we now travel with enough to get home in case our personal drive (our son!) can't pick us up. The final straw was a charge in Toronto that was a dup of the charge in Ottawa. Got it reversed but it showed to me that the fraud was far more organized than just the odd driver here or there, they were sending chits to other cities and having other cab companies running them through. In our case, it was a lucky slip of the pen that proved the chit was a duplicate of the legit one.

If I have to use a credit card, I'd write cab number, operator number and description of the trip on the chit.

On another thread about taxi drivers elsewhere on Flyertalk, one person always takes a photo of the cab driver's license when they get into the cab. They've found they get better service that way!