Just got a call from the obama campaign asking for a donation. No matter what candidate you believe in, donate!
Your time, money, whatever. I worked on the last campaign and it DOES make a difference.
Also, I can't recommend enough getting physically involved. My journey took me to a Nevada trailer park, and many days spent in an office in Culver City. You'll meet amazing people, and it's an incredibly positive experience, that I wouldn't trade for the world.
I found a very old blog entry from myspace (when I actually used to really write) about my trip to Nevada for Obama. You can read it here.