How to improve your ability to debug existing code

How to most effectively debug code?

Don’t assume anything.
Testing incrementally.
Write tests in the form “given”, “when”, “expect”. Write tests, don’t just sprinkle debugs.
Divide and conquer: try to identify visible input/output between which the issue exists. Try putting prints at sizable chunks or significant points between them.
What are the bugs you tend to write? It might be things like thread deadlocks.
Just assume things about semicolons.
Learn the nuances of your debugger and learn the hotkeys.
Write tests - TDD done right eliminates almost all trivial, stupid bugs, but also helps a lot in debugging. Testing forces your design to be modular, which makes isolating and replicating the problem a lot easier.
Learn the debugger. Talk to someone about your problem. Give yourself a time limit.
Is there really a bug?
If you have narrowed it down to two systems that collaborate, then determine which system is behaving to spec and which one is not.

Bugs happen for a logical reason.
Being confident on how this bug happens.
strace/stack trace. Being able to observe directly what a program is actually doing is incredibly valuable.
As I started being able to solve harder bugs, now when I find a thorny debugging problem, it’s way more exciting to me than writing new code.