Take the help of google.
Use google adword and other site like linkbuck to advertise your site :-)
If you dont want to spend money on Advertisement, then follow this ( I am running tons of site and this trick really helps):
create fake users and questions (wait ! no fake questions and answer, only fake users).
and then browse other Question and answer site, like answer.yahoo.com or stackoverflow, then copy question and answer to your sites, and remember to modify it little.
this will suerly increase google activity on your site. And dont forget social sites.