Competition helps but it depends on your auidence base which you are trying to target. If it is between 25-40, they are good to solve or answer trivia questions and hoping to get prizes but above the age range its tough. So you need to come with really good ones to intrigue your audience base and you should know them before you start one. Also as Fergal said, try to focus on quality not quantity to avoid some one timers who show for the prize alone. I would recommend writing blogs, visiting related websites and contributing them and collabrating with the webmasters to promote your websites. That would be way to go.