1. Look at the keywords that users are typing in to get to your site. Your webstats package (webalizer, web trends, whatever...) will have this for you.
2. Pore over the trade rags in your industry for buzzwords which are akin to problems that your potential clients may have which your purport to solve.
3. Get a subscription to Word Tracker (www.wordtracker.com) and run reports on all of the aforementioned words to get like words that wordtracker has determined are being typed in searches on the web.
4. Aggregate 1,2,3 into a test list for both Overture & Google. Your mileage will vary, by word, based on service. We're doing about 120 words and it never ceases to amaze me what works on Google and tanks on Overture's entire network of sites.
5. Focus on your copy - be exclusive. For example, one of our markets in the "residence hall" segment of colleges & universities. While "Dorm Furniture" would seem like a logical word choice, it's also the choice of dozens of college students looking for bean bags. So...we start our content phrases with things like the name of the decision maker, or with hot button issues for decision makers like "15 year warranty on...." to deter clicks from the wrong audience, and speak to the right decision makers. That's helped us a lot...
6. Plan, Do, See. Test, analyze, test, analyze, test, analyze.... repeat as necessary. It's a maddening thing to look at all those results every month/week/day/hour, but you NEED to focus heavy on the analytics or you'll be throwing away a lot of money.

Good luck!