Hmm, a single "most helpful tip" - that's tricky.
Okay, how about, "read a lot of code and write a lot of code".
Thing is, I really like programming, and would do it even if I wasn't paid. Early on, way back in the DOS days, I wrote most of the programs I used - contact manager, calendar, compy automation and so on, in C, then C++. Then when Windows came along I wrote the same things in Borland Delphi.
When I want to learn something, I spent a fair amount of time studying up front, but I learn way faster just starting to write something. Walking through with a debugger tells you a lot more than docs. Professionally it has been much the same. I work on a lot of legacy systems. I learn the code, usually by debugging, and then when asked to do something, I usually refactor to be more modern and reusable, and then do that reuse when possible. I'm not really a neophile so if I have a bunch of services I've used for 5 years that work great I'll just keep using them. Everything gets done with an eye for re-usability. I love solid code I can depend on.
Also, I keep really, really, stringent coding standards for myself, and the Dev team is mostly the same. We've agreed to these as a group, so we code review everything for SOLID, KISS, and conformity to our code review guidelines. It makes reading code a lot easier.
I hope that helps.