Best DIY tweak?

Ozzy.....I put some (Lowes teflon tape) on both ends of my speaker cables and interconnects(I put it on the center pins) and this took my system to a whole new level ! When you put this on all your cables(ac PLUGS TOO) in your whole system you will hear a difference.

Using the Ultra Bit Platinum liquid on your CD's.
Doesnt seem like it should make as much difference as it does.

I have made many compliations of my favorite artists. Ultra bitting the CD to copy from and the CDR (usually a Mam-Audio Master Gold CDR's) to be copied to.
The copy comes out much better than the original.With better soundstage, treble, midrange, bass.

I also will add that after buffing the heck out of the CD's, I then use my Bedini Clarifier on the CD's to prevent static build up before I copy them.

Knowing what you are doing or getting assistance from someone who does is the best approach. Tweaks are silly and you sound silly promoting them. What a snake pit.

Enid Lumley used to champion brass screws for mounting cartridges in tone arms before she left The Absolute Sound as a staff writer. Michael Green made some wonderful clamping stands that I encountered during my time in NYC. I built one using thick MDF found at an auction and discovered that it worked 'better than I thought it ought to,' certainly better than was reasonable.
At the time I had a partner who was not an audiophile. When I made a tweak and found she commented without knowing of the change, there was clearly merit to it.
So THE BEST TWEAK OF ALL TIME, bar none, though perhaps not the most affordable, IS AN OPEN MIND and the willingness to try something that seems unlikely, perhaps even something that seems ridiculous. I found that having another set of ears --preferably an experienced listener, as suggested above-- is both a great help and can forge very pleasant camaraderie.

All the Best, cdk84

This tweak business is all about creating an enhanced experience, right? Perhaps you're looking at the wrong end of the equation. Anybody who is so restless as to require constant variance in the sound delivery of their system will always pursue the moving carrot. Better I think to tweak the listener end than the system end.
A few years back when the Mpingo disc was the cure-all rage at the time, a guy responded to a question about its efficacy by saying he uses his every day and it never fails to make his system sound great. The trick, he said, is in how you use it. He put his on the arm of his listening chair under a tumbler containing 3 fingers of Scotch.