Targeted landing pages
Don't send users to your home page! Send them to a page that is optimized for the content of your ad.

Make broad topics narrow
If your website is all about cars, create PPC campaigns that are tightly themed to more specific content, for example "sports cars".

Split testing
Test, test, test. Changing one word can double your CTR! 

Conversion tracking
At minimum, use Google's conversion tracking code on your success/thank you pages. It is imperative that you be able to track conversions of keywords to sales!