Make yourself look good online. Know what you're talking about and make sense. On the net you won't be judged by the color of your skin, eyes, or hair, your weight, your age or your clothing. You will, however, be judged by the quality of your writing. Pay attention to the content of your writing.
Bad information propagates like wildfire on the net.
Be pleasant and polite. Don't use offensive language, and don't be confrontational for the sake of confrontation. Share your expert knowledge. The Internet itself was founded and grew because scientists wanted to share information. So do your part.