Test, test, then test some more
Oli Gardner has identified seven design principles that optimize landing pages for maximum conversions:
    1. attention,
    2. context,
    3. clarity,
    4. congruence,
    5. credibility,
    6. closing, and
    7. continuance.
It makes a nice punch list of the work to be done.
Article: How Good Design Increases Lead Conversions