Not sure if anyone else reads Mountain Bike Action, a US magazine. Lots of lovely bike pron to look at, and occasionally some really interesting articles too.
One such turned up recently, the idea of OBER. A natty little acronym standing for One Bag Each Ride. As in, each time you ride, take one bag with you to pick up some litter.
There will always be muppets who litter the trails out there, but perhaps if enough of us did OBER, it might make a real difference. I know that I've regularly shipped back a bag's worth on some of my rides, so I may just start doing it more! Every little helps, eh?