A point though is that people are more likely to litter if there is litter.  So an action "the choir" can take is to gratuitously pick up litter whenever you can.  I have done this on occasion even picking up other people's left dog poo if it happens to be close by my dog's leavings.  

Some people use this as their motivation to walk.    There is a YouTuber...Adrenaline Jackie who's theme is to collects litter on trails and areas she visits.  The more people pick up and are seen doing it the more the ignorant will catch on.  You certainly won't fix the problem since the self centered will always be with us,  but less trash left lying around is less trash left lying around.