It depends who you are aiming at and how to approach them as well.

Competitions can attract all sorts of people - but for your forums, the topics created initially should be topics that invite participation...opinions - a bit of controversy (not hatred etc), can spark some lively conversations and may help convert more people to registered users.

Ability to advertise or have sites reviewed would appeal to advertisers and webmasters - but you would need to enforce some basic rules (no one post advertisers, no spamming etc).

At the end of the day it will not always happen overnight, and can be a long process to try and establish loyal posters. Keeping the topics you personally create fresh and interesting will help convert more to members.

Adding onto what's been said...

Make the registration page as simple as possible.

Username, password, and email is all you really need for many forums. (Have your members fill out further details later if they want to enter a contest.)

I saw a forum once that had a simple bar where one just had to enter in their desired username, password (2x) and email and hit one button at the top of any page to register, I might do something like this. Plus have welcome messages at the top of all pages.