I don't know anything about being an "activist", but I prepared some remarks anyway. Then, when it came time for the panel, people were more interested in asking Sara Mayhew and me questions about our talks, so that's the way it went.
Since I prepared these, this is as good a place as any to record them:
1. It pays to complain. (title of a recurring column in Freethought Today): when you see church-state violations, or creationism in the public schools, or silly pseudoscience or outright scams, complain! Write a letter to the editor, or e-mail to the school board, or report scams to the police. You'd be surprised how much mileage you can get out of a single complaint.
2. Adopt your own style. You don't have to destroy a communion wafer to reach people. If you're comfortable with a more confrontational style, that's fine, but if you're not you can still have an effect.
3. Be scrupulous. You don't have to adopt the tactics of creationists. If you cite a quote, check it out first to make sure it's authentic. If you make a mistake, admit it. "Always do right," Mark Twain said, "This will gratify some people, and astonish the rest."
4. Ask hard questions. If your local elected representative has a meeting, go and ask how old he or she thinks the earth is. Ask their opinion of evolution and global warming. If they say something stupid, you can say "You are aware, I assume, that the scientific consensus is uniformly against you?"
5. Don't pay any attention to foolish detractors, whether they're atheists or not. No matter what you do, there will be critics; the "old school" of atheists like R. Joseph Hoffman are sometimes the silliest of all. Listen to people that have something valuable to contribute and ignore the rest.
6. Learn to be a good speaker. Record yourself and watch it. Watch videos of good speakers, such as Christopher Hitchens, and try to learn from them.
7. Know what you're getting into. Depending on where you live, speaking up might cost you your friends, subject your to attacks on you and your property, or get you fired. Choose your battles wisely! Not everything is worth your job.