Engaging tens of thousands of good-spirited people for a one day action sounds great, and also like a recipe for a big mess. There are many ways how to organize the action on the clean-up day, the main choice is how much of the co-ordination is done locally and how much centrally. Have a look at the presentation which explains the model tested in Estonia in 2008. This falls into more centralized approach, there are much more local approaches being tested for example in Portugal on 17th of April 2010. 

Presentation about the volunteer coordination model used in Estonia in 2008: