Mastering EQ

EQ the Midrange

Listen and try to identify any problems that you hear. Start with the midrange (vocals, guitar, midrange keyboard, etc.) as this will typically represent the heart and soul of the song. Does it sound too "muddy"? Too nasal? Too harsh? Compare it to another mix, perhaps a commercial CD. Try to describe to yourself what the difference is between the two mixes around the midrange.

Too muddy?
Try cutting between 100 to 300Hz

Too nasal sounding?
Try cutting between 250 to 1000 Hz.

Too harsh sounding?
This can be caused by frequencies in the range of 1000 to 3000 Hz. Try cutting this range a few dB.

Hopefully, using a band or two in these regions will give you a better sounding midrange.

You'll get the most natural sound using wide bands (Q less than 1.0). If you find yourself using too narrow a notch filter, or too much gain, you may be trying to fix something that EQ on a stereo mix can't fix. Go back to the individual tracks and try to isolate the problem that way.
Note also that the wider the band, in general the less gain you need to apply. In addition, your ears quickly get used to EQ changes. You may find yourself boosting more than necessary to hear the difference.

EQ the Bass

A reasonable use of EQ in the low end is to shelf filter below 30-40Hz. Purists might find this alarming, as yes, we can hear down to 20Hz and some musical information can be lost. Typically what people consider "bass" though is in the 50-100Hz region, and the audio in the 20-40 Hz range can usually be rolled off. The benefit is that you can remove some low frequency rumble and noise that could otherwise overload your levels.
Keep in mind that for bass, or any EQ change for that matter, every action has an opposite reaction. If you increase one frequency, you can mask another frequency. The flipside of this is that cutting one frequency can be perceived as a boost to another frequency. Each change that you make can affect the perception of the overall tonal balance of a whole.
Bass guitars and kick drums can span a wide frequency range. Where the "oomph" of the kick drum can be centered around 100Hz, the attack is usually found in the 1000-3000 Hz region.
Sometimes you can get a sharper sounding "bass" sound by focusing on the higher frequency attack, as opposed to the 100Hz region which can cause "mud". On the other hand, if you want to add that hip-hop style "ring" to the bass, try a peak at 50-60Hz.

EQ the Highs

Finally, take a listen to the higher end frequencies in your mix.

Don't be surprised if when comparing your mix to commercial CDs yours sounds a little dull or muffled. You could compensate for this with some high frequency EQ, with a low Q (wide bandwidth) band around 12-15kHz. Alternatively, you could skip the EQ and add some sparkle and shine using a multiband harmonic exciter.
Be careful boosting around 6000-8000 Hz. You can add some "presence" in this area, but you can also bring out an annoying sibilance or "ssss" sound in the vocals.
Noise reduction is a huge topic in itself, but you can sometimes reduce tape hiss or other noise by cutting high frequencies around 6000 to 10000 Hz. (You can also approach noise reduction using multiband gating, or dedicated noise reduction tools)
A generally pleasing tonal balance is a high frequency spectrum that rolls off gradually.

Summary of General EQ Tips

Try to cut bands instead of boosting them.
Cutting or boosting more than 5 dB means you probably have a problem that you can't fix from the stereo master. Go back to the multitrack mixing step.
Use as few bands as possible
Use gentle slopes (wide bandwidth, low Q)
Shelve below 30 Hz to get rid of low frequency rumble and noise.
Try using bass dynamics (i.e. multiband compression) instead of boosting low EQ if you're trying to add punch to the bass or kick.
Try bringing out instruments by boosting the attacks or harmonic frequencies of the instrument instead of just boosting their fundamental "lowest" frequency. If you try to bring out the fundamentals of every instrument your mix will just sound like mud.
Try using multiband harmonic excitation instead of boosting high EQ to add sparkle or shine. This, like everything in this guide, is purely subjective. Compare harmonic excitation to the effect of a gentle sloping EQ boost around 12-15 kHz.
Use your ears and your eyes. Compare to other mixes using both senses.

Have a look at a mastering EQ chart.

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