Getting involved in the political process is a very important one. Get active in the party organizations themselves and speak up when they are planning events, such as local state and national conventions. Just say “No” when they talk about having prayer invocations, and explain why.

Parties can only be changed from within, and if the parties are changed to me more secular, then the elected officials are more likely to accept changes to reduce the amount of public declarations of religion.

At least, that is my own approach. I am the Affirmative Action Officer for my local Democratic organization; as an atheist. No one objected to my election, so it can be done.
(It helps that the chair of our organization is also a secularist.)

“Two-thirds of Americans think religion is losing its influence on U.S. life, a sharp jump from just three years ago when Americans were nearly evenly split on the question, according to a new Gallup Poll.”

http://www.usatoday.com/news/religion/2008-12-26-faith-influence_N.htm