When I was running TGPs and MGPs, the techniques I used was buying skimmed traffic from similar sites (text or image based sites) and a small percentage of these visitors would bookmark the site if they liked the content. Obviously, fresh new galleries/content daily was key so to keep the surfer coming back for more.
In recent years, the set-up is very different what with the tube model but feeding traffic into a site is still key. Obviously, generating search traffic from the big SE`s is a big plus but very difficult nowadays in a massively saturated market.