Any guides/tips to write a good landing page?

Title says it all!

This is health related if it matters..

I always see these really BAD looking landing pages/sales pages, but so many people use them, so they must work? Right? But I really don't know how. How a normal person is not put off by the 1990's design, the ugly layout,flashing text, cheesy photos is beyond me.

Anyways, the Google multi variate testing tool is quite helping when testing different iterations of a landing page. You'd be surprised how 1 slight change can have a huge effect, so test test test!