Building An Online Community From Nothing?

Joe Jay Bee writes: "I'm currently trying to get a politics and current affairs discussion site (ad free link) off the ground, and finding it fairly hard; most places that would usually have users into such things don't allow links in signatures or recommendations, or indeed anything. So my question is this: given that I don't currently have a userbase, or any way to get one, how should I go about getting people interested in what I have to offer? If you've been in this sort of situation before, how did you manage to get off the ground? Have you got any tips? (beyond "Stick up some Google ads and make a mint", obviously.)"