I know this gets said a lot for these type of questions - but take heed cause it does really work
Go get yourself a hustler or maxim or some type of magazine that has ads targetting the male population - look at those ads - the pitch, the excitement and the "close" or call to action as Bill put it - and make damned sure that your free site "flows" into the sponsors site - there should be very little distraction caused by clicking on your banner and landing in the sponsors tour - same general colors are good - same type of "sell" that they use(although Ive seen some pretty lame sells used by sponsors lately) just make it seem the natural thing to do - or the "expected" thing to do - to click on their join link - dont tell the surfer that its the best site on the internet - they know better - just tell them how they're going to get their world rocked when they get inside that site
