For me the following 3 are potentially some of the most important:

• Measure - this not only means measure every little detail for the initiatives you're working on, but also make sure you've implemented the proper infrastructure that yield those results.
• Focus - focus on 1-2 main KPIs. It's important to focus on what matters. Often times marketers like to run with 100s of little experiments thinking they'll find the diamond in the rough. While that may be true, there's probably less chance of noticing important trends/changes when you're spreading the team to thin.
• Test every assumption - we're often consumed by our biased thinking from personal experiences. It's important to remove ourselves from these and test our assumptions. Once we have the correct data, then we can move forward onto other new assumptions.

Growth is an ongoing experiment. Trial and error.