EQ Guide

Note: EQ BEFORE compressing.

50 hz -
Increase to add more fullness to lowest frequency instruments like bass drums, toms, and the bass. Reduce to decrease the boominess of the bass and will increase overtones and the recognition of bass line in the mix. This is most often used on loud bass lines like rock.

70-100 hz 
To pick out the "weight" of the guitar cab.

100 hz -
Increase to add a harder bass sound to lowest frequency instruments.
Increase to add warmth to piano and horns.
Reduce to remove boom on guitars & increase clarity.

200 hz -
Increase to add fullness to vocals. Increase to add fullness to snare and guitar
Reduce to decrease muddiness of vocals or mid-range instruments.
Reduce to decrease gong sound of cymbals.

400 hz -
Increase to add clarity to bass lines especially when speakers are at low volume.
Reduce to decrease boxy sound of lower drums like bass drums and toms.
The "note" of the guitar

800 hz -
Increase for clarity and "punch" of bass.
Reduce to remove "cheap" sound of guitars.

1.5 Khz-
Increase for clearer, cleaner bass.
Reduce to remove dullness of guitars.
The "in-your-face" effect of guitars

3 Khz -
Increase for more attack of guitars.
Increase for more attack on low piano parts.
Increase for more clarity on voice.
Increase for more attack on the snare or other drums.
Reduce to increase breathy, soft sound on background vocals.
Reduce to disguise out-of-tune vocals and guitars.

4-6 Khz
The "bite area" of the guitar

5 Khz -
Increase for vocal presence.
Increase low frequency drum attack.
Increase attack of piano, acoustic guitar and brightness on guitars.
Reduce to soften thin or tinny guitar

7 Khz -
Increase to add attack to percussion instruments.
Reduce to decrease sibilance or that annoying ssss sound on singers.

8-10 Khz
"The air" of the guitar

10Khz -
Increase to brighten vocals.
Increase for slight brightness in acoustic guitar and piano.
Increase for hardness on cymbals.
Reduce also to reduce sibilance


15 Khz-
Increase this will pretty much brighten anything, but use sparingly as hiss and other nasties are here.

Overview
When a Q control is available, play with it to see what widths will give you the best results.
Remember, these are just general starting points, and by all means do not overdo it!
It is always better to cut to achieve the results you are after.