is doing your homework on the issues. There are a lot of groups out there who don't put enough work into their legislative agenda and proposals. If you want to advance your agenda, check out all the groups proposals and spend time reading the legislation as well as the position papers backing it up.
Good bills have to be both well crafted and backed by legislators who can get wide support in passing them.
Once you've chosen a bill to support, learn about how its going to move through Congress. A good advocacy group will help you follow it every step of the way and encourage grassroots action at each step.
Finally, build a relationship with your elected officials, whether D or R. Be positive and polite when communicating with them. If you've done your homework on an issue, they'll respect your opinion and even sometimes may call you for advice.
Another good source for advocacy "how to" is Congress.org
http://www.congress.org/congressorg/issues/basics/?style=legis