It's really helpful to train the ear on the different frequencies. It's a lot easier to use EQ when you know what you want to adjust is, say, in the 1-2k range than it is to play around somewhat randomly. A technique I used is to set a tight band (high Q setting), boost it like 10dB (that's a lot), and then sweep it around and listen to the exaggeration of different frequencies. This is an ongoing learning process for me, I don't claim to be an expert by any stretch of the imagination.
There's some vocabulary that you should spend some time learning, if you aren't already familiar with it. Frequency (in Hz) and decibels (dB) are your main parameters. The better you know them, the easier they are to use. Understanding the different types of EQ (parametric, shelving, high-pass/low-pass filters, graphic) helps in understanding overall EQ concepts. I found it to be a lot to absorb. I re-read things a half-dozen times and pick more each time I do. I was just plain dizzy when I first started to learn about it.
There's some good articles on prorec, and I'm pretty sure Ed has at least one on the main homerecording.com page. You can always find a lot by searching the forums here. Playin' with it is the best thing to do, of course.
In practical use, there's a few things I've learned the hard way:
* Listening to something with heavy EQ for an extended period will cause your ears to "get used to" the adjustments. You're brain will compensate for it, causing you to hear "normal" mixes all wrong. It's weird going back the next day and it doesn't sound remotely the same as the night before. Be sure to take breaks and occasionally check your ears with other reference material you know well.
* Things sound radically different solo'ed than they do in the mix. I think it's best to EQ with everything in the mix, since that's what you'll be hearing in the end.
* Cut as well as boost. Sometimes it's better to remove unwanted frequencies rather than just accent the ones you want.
* A little goes a long way. It's easy to over do it.
* They give you the ability to cut and boost a LOT for a reason. Sometimes subtlety isn't what you're after.
* No rules, it's your sound.
[Edited by pglewis on 09-12-2000 at 11:44]