In addition to the room and proper placement, cleanliness has been my single biggest tweak.
Its amazing how much better vinyl sounds after a good steam clean, scrubbing, and vacuum cleaning. A good stylus cleaner is essential too.
I clean my CDs with Pledge Anti-Static. A bunch of us at Ecoustics did this and everyone had great results. They ranged from subtle on new CDs to component upgrade like on dirtier discs.
I also check my connections every few months for oxidation. A little isopropyl alchol gets rid of somelight stuff, and some Caig De-Oxit works well on more serious stuff.
I haven't tried the high dollar specialty stuff. No desire to spend that much for probably not much if any difference.