It can be a book you read, a tool you started using or just a piece of advice someone gave you. What is the one thing that made your work as a designer significantly better?
For me, what recently improved my work was having an A3 notebook on my desk at all times. It's liberating to have a big space to sketch and write, and it helps get me in the flow much faster. Plus, the big pages help me keep a consistent documentation of what I'm working on.