It seems simple... place your ad on Google or Yahoo, set a budget based on your overall marketing budget and objectives. Then watch the clicks, web site traffic, and hopefully leads, roll in.

The problem is, there is little to nothing to prevent fake clicks. Security guru Bruce Schneier has an interesting article on Wired about this very issue, which is causing Google to rethink its fraud prevention mechanisms after numerous lawsuits.

http://www.wired.com/news/columns/1,71370-0.html