I do art trades fairly frequently. Usually I approach another artist about it if I see they're open for trades and I like their drawing style. I prefer to trade with people who are about on the same level as me as far as the art quality goes, but other things can sometimes be arranged. I don't like having to judge people's art as being better or worse than mine, but it's not fair to exchange art that's vastly different in quality. But sometimes I do it anyway just because I have fun with it.