Google AdWords includes a wonderful, often unused feature for negative keywords. This allows you to filter out the unwanted irrelevant traffic that would still cost you money in clicks, but not gain you anything in sales.
Using the example of a campaign selling hubcaps, if you don’t sell Renault hubcaps, make sure you use “-Renault” in your campaign. If you provide a service in a particular geographic area, get a list of all of the places outside of that area and include them as negative keywords.
Using Google, search for your keywords and see what results come up. Retry your search using search term -keyword -keyword -keyword in the search box, keep applying this until all of the irrelevant results are filtered from your search. Whatever negative keywords are now included in your search query, add to your ad groups or campaigns so that others users searching or those irrelevant terms don’t see your ad.
This improves your quality score, and therefore the cost per click for your ad or improves your position. The improvement in your position should increase your click through rate (CTR). The increase in click through rate is rewarded with a better quality score, which again then should reduce your cost per click.
It pays to be relevant.