The problem with getting traffic to ones site, is keeping that traffic there for a long time. You want to convert every visitor to a new user, but that usually is never the case. It's one reason I don't like to purchase traffic for sites anymore, as they provide a boost in activity for a little while, and then that's it. Usually they don't stick around.
So, what are your methods for keeping people staying on your sites? I think doing a giveaway is a good option, but that also results in the same situation, where they show up, enter and then leave. What do you do to keep people around?