Selecting the keywords for your site is time consuming task. Well, you have to concentrate a lot on keywords, because they are the ones that help your site to climb the rankings of the search engines along with the back links. But, how should we choose the keywords? One way of doing this; find a site similar to yours that has a good page rank. View the source code and copy the keywords. WRONG, WRONG, WRONG. Bidding for the same keywords as high ranking sites is completely wrong strategy. That would not get you anywhere near the first few pages of the major search engines.
You have to do some research, when you are choosing the keywords. Find out, what keywords describe your web site well, what keywords are original to your web site. Let me give you an example: "job search" is a popular keyword. There has been 11,719,478 searches according to Keyword Discovery. However, "job search site" has 44,085 searches according to same tool. This means that by bidding for job search keyword, you have less chance to be found in the search engines. On the other hand job search site appears to be less popular keyword and more chance to be found by the search engines.
Above example doesn't mean to stick job search site all over your web page. What it means, grab yourself a keyword tool such as Wordtracker and Keyword Discovery, and start researching on the keywords for your site. Bid for the keywords that has less searches and describes your site.