Get the right facts and information at hand before you start your work. Doing interviews with customers, salespeople and support agents will allow you to understand pain points, persuasive angles and what success means to everyone involved. It’s also critical to define what key metrics are used internally - to make sure you are designing against them. There is no better way to lose credibility than working for weeks on a UX project that has no impact on the bottom line.
In 10 years of growing businesses, usability testing has always been my ultimate research method since it consistently gives the most actionable insights. However, if you are using online services like usertesting.com do not fall into the trap of listening ‘too much’ to your testers’ opinions e.g. "I would not buy this", “I would remove this” (since they are not ‘qualified' to do so) - instead focus on what they do. Where they stuck on a page, did they miss a button or key part of the content? Alternatively, power up your usability testing by recruiting your own visitors for tests.
Note: Always keep in mind that one user’s opinion is not the equivalent of a split test result (where you get a statistical ‘vote’ from your users – see point 7 below).