The general approach to conversion optimization is to consider that every ask you make will involve having to address user anxiety. Work hard at understanding the mindset of the user and what fears they may have at each stage of the funnel, each form field, and in each email. Test out copy and images to see what works best to address any fears.
Also dig into the performance of different user segments such as traffic sources as each could represent a different type of user with different need and fears.
A pro tip: when testing, if you don’t see a strong signal within a few days (assuming you have the traffic), your consider making your variations more different. By testing extreme variations, you have a better chance to find a global maximum.