Are you using AdWords from Google? You need to read and understand the points in this excellent article on AdWords. Advertising on either side of the coin.
Excerpt: Here then are seven common mistakes to avoid when setting up and optimizing an AdWords campaign. Steering clear of these potholes and using SEM best practices greatly increases the odds of success with AdWords.
Read full article via Seven Common AdWords Mistakes to Avoid | B2B Marketing Blog | Webbiquity.