Okay so here's an idea:
The other board I belong to has t-shirts and window stickers for your car (it's a car forum) that say 'member' and have the board's site on them. Nothing flashy, not ugly but they get their point across. I don't have a t-shirt, but a sticker on my car. It appeals to those who share your interests, if someone was looking at my car (interested), saw the sticker and could log-on to the site. Could work for HB, someone see's your Husker hat, and sees you’re wearing a HB t-shirt, is interested and logs on.