Lots of great recommendations in this thread. The best way to think of the process is to look at your ads from Google's perspective. Google wants to make money right?
Well, many mistakenly believe that this simply means that Google will jump on the ads with the highest bids to make the most "per click." It's more complex than that, as many times, Google makes more money by showing the ads with the lower bids.
How so? As others mentioned, higher click through. If you have one ad with a bid of 1.50 per click, and a click through ratio of 3% squaring off with another ad bidding 2.00 per click, but a click through of only 1%, which are you going to make more money from? (Remember, you're Google still - haha)
The best way to lower your costs is to keep improving your click through ratio, and focusing on the relevancy of your ad groups, as Google is going to assign you a Quality Score for each ad/keyword combo.
To better match up and group your ads/keywords, I recommend using the Google Wonder Wheel to get an idea of what groupings Google considers relevant. Chances are, if the Wonder Wheel shows the groups as belonging together, you should also group them together. (Not always though, it's up to you to apply common sense and a human understanding to what ads make sense when grouped with specific keywords)
Beyond this, make sure your landing page is also well done. By this I mean that you should have your proper keywords placed on the page, and have your proper links/disclaimers on the page, etc. Single page sites that are light on content and relevancy are getting punished by Google, as they are fed up with "splog" type sites.
Earn a higher QS from Google by demonstrating you are NOT a splog.
Best of luck to you.