They leave an unforgettable impression. Not everyone wants to do the work involved in this one, but it can be a significant factor in developing both name recognition and reputation. You can do this by your sense of humor, the strength and uniqueness of your comments, your outrageous style, or any other approach that clearly distinguishes you from the average online fare. They achieve persistent, targeted visibility. Two elements there.
Persistence is a big one.
You don't get a reputation that's worth having by posting once a month. It's not necessary to post every day, but it can't hurt. To really develop a reputation fast, you should post whenever you have something really useful to say. A useful way to measure this is to keep a list of forums you want to post to, and a log of each post you make. Review this weekly.