I have to admit I learned this from the electronic music poster boy known as BT http://en.wikipedia.org/wiki/BT_%28musician%29

In an interview with BT, he revealed one of his 'secrets' which even he admitted wasn't Earth shattering - he was just amazed that most people don't employ this simple trick. It goes something like this: Except for bass and kick drum tracks, there really aren't too many tracks that don't benefit from rolling off everything below 120 Hz or so. Most hardware mixers even have a HPF switch you can throw in (roll-off is usually lower, but same principle applies).

A simple High-pass filter placed on most tracks, with the cutoff coming in anywhere between 100 and 200 Hz or so can do a heap of good to tighten up the overall sound of the whole mix. Strummed acoustic guitars can even go higher if you are just trying to get the rhythm of the acoustic to sit nicely in the mix. Same with vocals - also tends to eliminate some of the severity of pops and such without having to rely upon compression.

-Scott

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