Apart from the direct investment into the traffic - take a look at Chameleon Traffic Solution Tools.
Pay sites use it in-house as a way to promote their sites. Also, good way is letting affiliates use the traffic solution tools and have them work for you this way..
Check out - Chameleon Submitter (
http://www.chameleonsubmitter.com/as.php) , Chameleon Confirmer (
http://www.chameleonsubmitter.com/ac.php) , and Chameleon Partner Account Requester (
http://www.chameleonsubmitter.com/ar.php) .