SPAM. I spam the hell out of people...
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Just kidding!
We spend between 10 and 30 dollars per day on PPC, and have for the past 2 months. It keeps traffic steady.
At one time, we were getting around 2000 to 2500 unique visitors per day, then we had a really long downtime, like a year, after we changed the site almost totally, changed servers twice, changed operating systems, html to php, etc. etc. and we are finally recovering from it.
We will be getting a free add pretty soon in a very large nursing magazine thanks to a partnership in the works with a major association, and are getting ready to launch a print ad.
Now, we are working with the SEO specialist from Webposition Gold, to improve our search engine rankings.
We just bought a ton of pens to send out to hospitals and other companies as well. Nurses ALWAYS need pens and seeing a pen lieing on the nurses station desk with
www.ultimatenurse.com on it can't hurt. A quality pen is a pretty valuable tool to a nurse

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Anyway, the best suggestion I could probably make to you for increased traffic, AND ad revenue is this:
Buy another copy of threads, give it away to a site that has good traffic but not a good forum, and have it use the same database. They get a quality forum, and it appears that there is instant participation, AND in return, they have to display your google adsense ads on the forum so you make money.
Plus, you own the database so if they leave, they leave with an empty forum and user list. Therefore, you have the control, and once people start signing up, they pretty much have to continue with you...
Then you are even more virtual than you were before (if that is possible) because site visitors can now discuss issues in real time with site users from other websites...
I've done this with my nursing forum as you can see from the links below:
http://www.ultimatenurse.com/ubbthreads/ubbthreads.phpand
http://www.nurse-recruiter.com/ubbthreads/ubbthreads.phpand
http://www.nurseserver.com/ubbthreads/ubbthreads.php there are 7 more in addition to those but you get the point.
I'm also integrating an instant messenger solution only for nurses from
www.interactiveni.com which will use the same user db. So all users have an IM solution that interacts with yahoo, aim, msn, icq, etc. and anyone who signs up for the IM solution will have a forum user/pass as well.
I also have an pretty substantial email newsletter subscription...