Your most indispensable "tweak"?

Trying to narrow my possibilities down... 
+1 with atmosphere & mrdecible. The majority mentioned should not be considered a tweek in my opinion.

A tweekers system will always sound better then a plug & play guys system and once you enter the high end hobby everything matters especially entry level not just the super high end.

I agree weed is a good tweek.
George, do you also have PPT "Gate?"
Do you use them together?  What is the strength
of the "box?"  How does it work?
nonoise, Where is the most noticeble impact of the "oreo", turntable, preamp amp, CD player ?
nonoise, I meant: Iso acoustic  Orea, sorry .

i used 4 oreos as isolating feet under my dac and tt

it definitely sweetened the sound... more than a little  ...  ;)