iMedia has a great article showcasing what makes a site sticky.

All things being equal, the longer people interact with you, the better a shot you have at establishing and/or deepening your relationship with them.  IMedia provides some great examples and ends with a detailed checklist of tactics worth trying.
One word of caution — the article is irritatingly old school in its presentation, making you load page after page to read the whole thing. Very 1999 in the way it makes you rack up page views for its advertisers. It’s worth it, but they ought to know better.