Remember this fundamental. People search for keywords -> See your ad that interests them -> See your salesletter -> Buys the product.

Your focus should be on the first 2 steps which is determining the keywords and crafting the ad. Different keywords, Different Ads. The more congruent the flow, the better the conversion for sales will be.

You want to be specific and not be everything to everyone. This way you can be sure to get your target audience who are looking for your product.

A typical conversion for a salesletter is between 1-2% so if you are falling short, you will want to revise the keywords and ads that you are running to get better results.

Here’s a great resource on how to effectively use Google Adwords, a large part which also applies to other pay per click search engines as well.

AdWords Learning Center
http://www.google.com/adwords/learningcenter