Advices
This is only my subjective opinion that came from my experience. They probably change in a future, but there is no commitment from my side to update this advices if I learn that they are wrong. Be careful using them, because this advices can be bad in your projects.
Solve the problem
Main purpose is of our development is to deliver solution that solve problem. To provide proper solution, we need to understand the problem. To understand problem we need talk. With client, with users, to learn about the process, how they work, to find the real issue. Client and users know a lot about problem that they have but their/our solution can be for effect, not for cause. And instead of review a process, we can add additional level of complexity.
Experience over Interface (UX > UI)
Applications that you create can look great, but if the user does not need half of what is on the screen to do his job, what is the point of looking great if user waste his job time in app do simple tasks.
Better done than perfect
You should focus on delivering a solution for a problem, even if you see that something can be improved, because working solution can be tested in a real environment and some features can be used in edge cases, and user can just not care about it.