These have to be some of the best tips I've seen on the subject!
Excellent piece by searchenginejournal.com:
7 Tips To Increase Landing Page Conversions
1. Call to Actions Matter
Have an offer on your landing pages and tell your visitors what action you want them to take. “Fill out the form below to Run Faster, Jump Higher and get Hot Chicks” is not overkill. Adding a call to action where there was none to your landing page, whether it be a high ranking subpage or a ppc landing page will have a drastic impact.
2. Digestible content