Most pay per click advertisers that find themselves at a point where they are profitting off of their ads eventually reach a point where they want to add new keywords and phrases to their campaigns. Whil this is a good idea, the way that you go about doing it can make a big difference in the ongoing success of your campaigns. To that end, ClickZ's Kevin Lee is just starting a two part article on Keyword Testing and Expansion that's worth checking out.

From the article:

Keyword expansion gets a lot of attention. If you find a new keyword and it can deliver significant incremental high-quality traffic to your site, your business will benefit for as long as it remains a profitable part of your overall campaign. There are right and wrong ways to add keywords to a campaign, however, because every keyword, new or old, must be tested to determine if it can become an important profit driver for your business. If it can, further testing helps determine how best to maximize the positive impact each keyword has on your business.