Try out different designs, switch around and modify specific design elements, and track conversion/clickthrough differences for all changes.
Unless you already have the perfect design, simply experimenting a bit, comparing the results and optimizing based on those results will usually net you 10-30% higher profits for any given site.
It's been bugging me for a while how few of the smaller webmasters and programs appear to be doing this, especially because it's been standard practice in large parts of mainstream for years now.
Simply buying a nice-looking design is not enough, you need to experiment and optimize to get results
