From the folks at Link Building Blog (the blog for Text Link Ads):
Assuming that you already have a good idea of what keywords you want to optimize your site for you can take that list and enter it into your Google AdWords account. I recommend using a combination of both exact match and broad match. The reason I recommend using broad match is so you can find the keywords that people are searching for that you may not have thought of before. To do this accurately though you will need to watch your referrers in your server logs and analytics software. And once you have some good data from broad keywords you’ll need to enter those in as exact match so you can get exact data.