Know the facts. It's always a good idea to be intellectually armed. Since our way of thinking is based in facts, not myth or faith, we need facts that can be proven at our disposal at all times. I am guilty of not being armed as well as I should. Facts are what we deal in.
Science is our bible.
Ideally, if you can't prove it, don't say it. Spread the word. If you teach them anything else, make sure that they understand that we claim nothing but a lack of faith.