Are There Improvements that Can Be Easily and Quickly Made Without Buying More Stuff?

I appreciate that there are many people on this forum who have put a great deal of thought and effort into how to improve their audio systems.  Most of the discussion relates to upgrading equipment.  This is natural as most people here want to improve their music experience, and better equipment is a way to do that.  I’ve taken advantage of this and made 5 or 6 upgrades over the last year!  The result has been great!  But are there some simple steps that can be taken that can be overlooked by someone who is newer to audio?

As examples I’ve read that facing a sub towards something like the back of a couch will improve its sound.  And for rooms with bad effects that can’t be fixed, move the listening position closer to the speakers.

‘What is your best tip for improving sound quality without buying anything?