Why is voting insufficient for citizens to affect public policy and what else can we do to get our voices heard?

Just voting doesn't have much impact, voting along with letters/calls to Congress, showing up at town meetings and party meetings, letters to editors, etc, have a fairly big impact. A representative government takes work on the part of educated and motivated citizens, someone who hangs around for four years and decides at the last minute to vote for a presidential candidate is not actually involved in the process

what else can you do ? RUN FOR LOCAL OFFICE--council, township trustee, central committee, board of education. GET INVOLVED, MAKE A DIFFERENCE, be a part of the solution instead of a part of the problem.

It's insufficient because we all subscribe to the team 1 vs team 2 groupthink. Teams 1 and 2 have candidates favored by lobbyists. Lobbyists are the ones with big pockets. Both teams 1 and 2 like the lobbyists because they like money. Politicians are employees of the people. The "people" who matter to them, though, are the lobbyists because regular people like you and I aren't filling their pockets.

edit: I didn't answer the latter part of your question...

We can get money out of politics. Until then, our government will only serve the people with money. It's hard to fight rich people though, so we're pretty screwed! The only real thing we can do is get used to the citizen losing and learn to cope.