Everyone has different Formulas and Reasons for using Landing Pages whether for Google Adwords etc., so it all comes down to Testing... Marketing and more Marketing!

Here are a few Excellent Landing Page resources worth Reading and Studying.

Mostly Free, although a few links from within may have expired...

And of course there's Lots more out there...

http://www.marketingexperiments.com/...imization.html
http://www.uie.com/brainsparks/2006/...er-on-promise/
http://www.grokdotcom.com/topics/landingpage.htm
http://library.mequoda.com/tocs/lph2.html

"FREE" membership and well worth the investment.
http://www.marketingexperiments.com/...s-account.html

Phil