I spend part of my time in a city and have challenged myself to pick up one bottle/can a day.  Even though I don't use single serve myself, I figure I can counteract at least one litterbug.  

Extra points for those bottles that have a 5 cent deposit!  It doesn't seem that a penny is worth it for most people these days, but would you pick up a nickel?  $15-20 per year.  Better than the interest you most likely earn in the bank these days.