In very brief, focus focus focus, on the product of course. Make it feel like a funnel, pushing the visitor to the purchase, with no distractions such as links and unrelated content. And the usual techniques such as discounts, testimonials, happy people, and so on... It also depends on the page visitors come from, if there's already a lot of info there, don't put too much on the landing page.
I can be varied depending on your goal. Promoting affiliate program? Getting subscribers? Generally you want your visitors to focus on one specific action on your landing page.
Several things, but the main thing I did was to put the ordering form on the sales page, down on the bottom. Hit them at the heat of the moment.