As a dog owner myself I can attest to the increasing amount of of poo not being picked up. One thing I would suggest is that more bins should be provided along popular dog walks and not just at the start and end points. Ours loves Portrack Marshes, but there is only one bin at the Asda end and nothing else until you get down to the Talpore, having more bins would encourage people to carry the bags rather than sling them and if it was still a problem, remove any excuse to get out of fines.
Overall, I think litter is just another manifestation of the general lack of respect people have for themselves, others and the environment. Education and fines need to be increased massively and technology brought into make it easier to spot offenders.