Here are three very simple things to follow:
1. Clearly state your purpose.
2. Simply display your promotion.
3. Make the opt-in easy.
There are a lot of various factors that come into play as well, but these three items are basic essentials. Other factors include design, content, and ease of usability. Overall design (the look and feel) of your landing page is very important. Everything should be complimentary of each other- from color schematics to flow of copy to ease of signup. If you are using a PPC, your ad copy should be reflected into your landing page to keep this easy flow going and keep the user interacting with you. One other big thing we have taken from our experiences is to TEST, TEST, TEST! Testing is the best way to understand what works and what fails. Test and redesign continually until you find the optimized landing page that works.