Basically, anything that relates to something on your board. If it's a general board, go to places where people who would be interested go.

Like, for a computer site, it could be beneficial to advertise on web design pages, as they'll be interested in computing things.

All you gotta do is figure out what people on your forum are also interested in, and advertise on a site like that.

If that made any sense whatsoever, you're a clever person.