I think that in order to produce your best work you need to be honest with your approach towards what you do and what you are. Having general principles or guidelines is healthy and they should evolve over time but maintain an essence that defines you as a designer. These are mine:
“Great art makes you wonder, great design makes things clear” - John Maeda.
When designing we are producing things people will use to get something else done, something important and we probably will never know what it is. Therefore we are not creating a piece of Art – something to be looked at and admired in all it's beauty. We design to make things clear and easy to use, that is the essence of the product because it's means to an end. A well designed product will always be beautiful.
Design is the details.
There seems to be a level of conformity with MVP's (Minimum viable products) and the philosophy of test early, fail fast and learn from the results. While I think some sort of that is necessary, I believe great products were created while going the extra mile. Be obsessed with your best supporter and your worst critic, never call it a day with anything you design, it can always be better. That's what differenciates great from good.
Reuse, recycle and create something new.
A designer should not reinvent the wheel every time, but use native components on apps and embrace the fluidity of the web on sites. Use the affordances provided by best practices stablished in each platform. It's very likely that others have faced a similar problem. Research and reuse, tweak and adapt. And most of all follow the rules first and deviate later. As Dylan Thomas said "In order to break the rules, first you must follow them.”
Data informed, not data led.
Nowadays, there’s a ton of data sources that a designer can use to improve a product: analytics, marketing reports or metrics for every measurable action. Quantitative data is very valuable as a way to point to a direction or reflect that there's a problem. But it should be used just for that. Avoid jumping quickly to conclusions, investigate further and use data to inform later qualitative research.
“Specialization is for insects” - Robert A. Heinlein.
This is my personal and biased view on design and, so please take it with a pinch of salt. I know in every organisation you need specialists and generalists. But designers should per se be curious about the environment they are working on. And that means learning business skills, code, product management or you name it. Learning never hurt anyone and as Heilein said "A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship..."