This site makes it easy to contact your government officials at the national level.
Politicians don't have time to read every letter or email, but their staffers tally up the number of people for and against each issue. For every constituent who contacts them, they figure there are many more who feel the same way.
I've heard that if you take the time to write a handwritten, thoughtful letter, that is most effective. But that can be so daunting that most people won't even do it.
Therefore, I recommend jotting down issues and keeping a text document handy to list them. Later, when you feel like it, write your officials, devoting just one line to each issue, saying whether you are for or against it.
As a Green I am cynical about how effective this all is, but it can be cathartic to at least go on the record, and sometimes it really does work!