A group of volunteers from our marina (Mercia) have adopted a length of towpath, about a mile, between the marina and Willington village. We have regular litter picks on that length in addition to various improvement projects in association with C&RT. Since the litter picks started the condition of the towpath with respect to litter has significantly improved. This supports the argument that litter attracts litter.
In general I prefer Maffi's approach but it certainly seems to help here that people are seeing the volunteers at work and commenting on the work being done. And our group is mainly boaters.
To give C&RT their due, they are promoting the idea of local groups adopting lengths of the Trent and Mersey and tidying them up. It strikes me as a very good way of using volunteers as I quite see that C&RT haven't the money to get the work done commercially.