What can you do? The marine litter problem is global and belongs to everyone here's a great place to start looking for answers.
NOAA—Check out the U.S. government agency's Marine Debris 101 Resources:
http://marinedebris.noaa.gov/marinedebris101/resource.html
Also, NOAA's What Can You Do?
http://marinedebris.noaa.gov/marinedebris101/whatcando.html
Ocean Conservancy Join the International Coastal Cleanup:
http://www.oceanconservancy.org/site/PageServer?pagename=icc_home
Keep America Beautiful (KAB) has chapters across the country that encourage citizens to fight the scourge of litter:
http://www.kab.org/
United Nations Environmental Program (UNEP)—Check out UNEP's Global Marine Litter Information Gateway with anti-litter ideas and a special kids section:
http://marine-litter.gpa.unep.org/facts/facts.htm
NOAA/University of Washington 2008 workshop:
http://marinedebris.noaa.gov/projects/rschwkshop.html#proceedings