It's a mixture of a lot of things. Katz bring a lot of traffic but not that many convert to members IMAO. If you want members I find you get more by uploading good quality movies, apps, games etc with a password and info file. Then post them around but make sure their good quality posts with screenshots etc. When thoes people come their far more likely to register and if the original download is good quality they'll return. SEO, link exchanges and site maps are also important but patients is the key.

EDIT: and don't be tempted to leech posts to make you site look full. If it's quality people will register not quantity.