I think that in order to produce your best work you need to be honest with your approach towards what you do and who 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 we design, we are producing things people will use to get something done, something important and we probably will never know what it is. Therefore we are not creating a piece of Art; a design to be looked at and admired, we design to make things clear and easy to use. That is the essence of the product because it's means to an end. And a well designed product will always be beautiful.
Design is the details.
There seems to be some level of conformity nowadays with MVP's (Minimum viable products) and the philosophy of test early, fail fast and iterate from the results. While I think some sort of that is necessary, I believe that great products were created while going the extra mile. When you are obsessed and bring all the best in you. 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 websites. Use the affordances provided by best practices established for each platform. It's very likely that others have faced a similar problem before; do your research and reuse things with no fear, but don't blindly copy – always tweak and improve. Follow the rules first and deviate later, as Dylan Thomas said: "In order to break the rules, first you must follow them.”
Be data informed, not data led.
Nowadays, there’s a ton of data sources that a designer can use to improve a product: analytics, custom BI 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. I always try to avoid jumping quickly to conclusions, and use that direction to investigate further and inform later qualitative research.
“Specialization is for insects” - Robert A. Heinlein.
This is my personal and probably biased view on what a designer is, so please take it with a pinch of salt. I know that in every organisation you need specialists and generalists. But I belive designers should always be curious about the environment they are working on. And that means that besides design, there is also room for learning business skills, code, product management or you name it. Learning never hurt anyone and as Heinlein said "A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship..."