How much do you optimize your landing pages?

Just a quick question for readers of this blog, if you have a moment I would love to hear your experiences. How much time / effort do you spend optimizing and testing landing pages? Do you only optimize PPC / E-mail landing pages? or do you optimize every page thinking it could be found through natural search?

Additionally, would you read or be interested in a series of articles that discusses methods optimizing landing pages and testing them against each other?