Keyword research is something that is crucial to the success of your campaign. The main thing you have to realize is you can't be afraid to change your strategy after you launch your campaign. SEOBook has a good list of resources Free Keyword Suggestion, Research & Analysis Tools: Seo Book.com Depending on the type of site you are launching you may also want to check out Google Hot Trends to help you find out some of the most popular keywords

I have used both WordTracker (WT) and Keyword Discovery (KD) and I decided to use WT. KD seemed to have some problems with returning results and other minor programming glitches. I did a comparison of the results from both WT and KD and they were fairly similar in amount of traffic but keep in mind the numbers they give are estimates and may not be 100% accurate.

Once you have your campaign launched you may want to look into installing HitTail to help you determine some long tail keyword phrases that might be worth your time. Of course Google Analytics is a good resource for finding out how people are getting to your site. Once you know how people are finding you do more research on words that are related to that topic.