I've been working extensively on our Google Adwords keyword list at work. I use our Omniture stats to look at the list of keywords used to find our site. It gives me ideas on what people want to know. If I search on “How to” then I get ideas for articles to write. I also search on “what” because those are often questions too.
Another way I search is for possible negative keywords. Since we're an enterprise level company we want to eliminate words that suggest a free or consumer product. Or simply words that don't apply. That way we're not wasting money.
I also use the day parting feature on Google to turn off our ads during the night and on weekends when we get low quality clicks. The clicks were often for other countries. Not many administrators are searching for a networking solution during the middle of the night!
Long Tail Keywords
I also use web analytics to find the long tail keywords. These are keywords that are low volume but have less competition and are often very specific. You'll probably notice when you look at your keywords a few branded terms account for a good percentage of the total keywords. In my case, around 30% come from maybe 4 terms. Then there is the long tail. These make up about 60% of the total keywords searched on to find our web site. Most of them only have 1-3 searches.
At my last job my boss told me to ignor those because they didn't pay enough to warrant attention! Wrong. Those are the gold in your keyword list. Why? Because they often cost next to nothing and they often have little or no competition. True, you may not get the volume of searches, but if they are so specific that they convert well, who cares?
Save Your Cash for Keywords that Get Results
Pay attention to the keywords that people are already using to find your site. It can tell you a lot about your potential customers. You know these people came to your site. Why? It can give you ideas on what to write about. It can give you ideas on what keywords to exclude. Also, don't ignor the key words or phrases that get low volume but are inexpensive. They may convert to sales quite nicely.