giveaways are only good when you're trying to attract new customers to the site so it'd be best done when your spending quite abit of money promoting the site.
Hm...you did bring up an extremely good point. I know for sure that contest/giveaways will definitely attract many new visitors to this forum and they can start posting!
Giveaways are great, people are more active and you'll get more members since people probably tells them to join to get the giveaways.
I've tried it sometimes and yes it worked very well.
I used domains, hosting, graphics and money as giveaways.
You guys should have a free random domain contest every month, each month a random user wins a domain or you can have a post count or something of the kind.
I know my friend does it on his hosting sites which generates a few bit of new users.