What are your favorite websites for learning about or building effective landing pages for online advertising? How should these pages relate to your main site?
I'm a software programming geek that needs to get hip to this marketing thing.
One of the best sites around right is Smashing Magazine. Here's a resource on Conversion Rates and Landing Pages turned up by 5 seconds on their site. There are definitely more.
Other great places to look are A List Apart, The Google Adwords Blog (linked to a useful example), and of course make use of Google Webmaster Central.
I discovered a great series of articles on Copyblogger shortly after posting this question:
Landing Pages Turn Traffic into Money The articles include insights on both the wording of the page and its design.
What you want is A/B testing, as Usman mentioned. In essence you build two different versions of the page and measure which one performs better.
My suggestion is the Google Website Optimizer, and related book Always Be Testing
I like Unbounce for landing page tools and tips.