I use newspaper. It's thinner than printer paper but still keeps the shape, is cheap, and is flexible when wet so is very easy to peel off. Do it while the pot is still wet and the rim can stick to the newspaper.
Clear off any extra slip on the rim and then gently lay the paper on top. Make sure to get contact all the way around the rim by very gently running your finger along the rim (you'll see where it is sticking) . Move the pot off the wheel onto your ware board and then gently peel off the paper. You may have to smooth the rim a bit if it looks rough, but it will be perfectly round!