Interest or advocacy groups have come to have an increasing impact since the 1960s. Traditional business and trade associations work to protect and advance their members' interests. Political consultants advise elected officials, candidates, parties, interest groups and associations on how best to present their arguments. The growing professionalization of politics in the past fifty or so years has contributed to today's costly campaigns and may be one factor alienating some voters from the political process. Foundations provide grants that fund think tank studies, some fairly extensive media campaigns, and specialized news coverage.
News organizations influence the agenda by what they cover and how they cover it.
Most importantly, individual citizens such as you rself, taking an active role in your neighborhood or community, make our democracy work.