I usually thank the bus driver, just with a "Thanks". Mind you, in Cardiff, they come in for a lot of abuse including being stoned and shot, so I feel sorry for them.
I have more contact with bus drivers than is good... too much public transport usage! And a "cheers" or "thanks" usually suffices, 'cause generally they're very moody. I've only ever met one nice one, and he was actually a scheduler/office worker or summat who'd been drafted in to drive the last bus of the day. Still, he gave me a lift about 8 miles off-route and didn't crash into anything, so I'm not complaining.
I once got on a local bus in Pontypridd, driven by the usual driver, who was very nice ordinarily. Unfortunately he had just broken up with his girlfriend and seemed to have a deathwish - as we drove along some roads with very steep drops without stopping the whole trip it was a wee bit scary..
In Edinburgh the whole 'to-thank-or-not-to-thank' dilemma is solved by having exit doors in the middle of the bus. Communication with the drivers seems to be strictly discouraged, in fact, by virtue of them sitting in a perspex isobooth. Your fare is poured into a slot, whereupon the driver presses a button to signal its descent down the chute of destiny into the locked safe of eternity (they don't, famously, give change). The ticket appears from a hole in the wall. The whole thing works so that even eye contact with the driver is a near impossibility... I quite like it that way.