Equalizers can be a formidable tool in the studio, and therein lies the danger. Often, novice engineers get lazy when recording tracks, thinking that any inadequacies in the sound of an instrument can be fixed by the use of EQ when mixing. The fact is that EQ isn't a miracle worker, and a badly recorded signal often can't be salvaged. It's best to think of EQ as a way to fine-tune rather than to fix a mix, and it's wise to spend more time during tracking to ensure the sound being recorded sounds as good as it can be without use of EQ. Making sure instruments are correctly tuned and that the appropriate microphones are chosen and optimally positioned will make many EQ adjustments unnecessary.
When using your EQ controls, don't automatically boost a frequency to make an instrument cut through a mix. Adding EQ to a mix also adds noise, so you'll have to continually adjust the gain control to avoid the digital distortion known as "clipping." If, for example, you're boosting guitars, vocals and keyboards in the 3kHz range in an attempt to make them cut through the clutter and be more audible in the mix, you'll be reducing the amount of available headroom before the signals clip and distort. On top of that, this is a frequency that human ears are quite sensitive to, so boosting this frequency on several instruments can easily result in an ear fatigue-inducing mix that quickly becomes unpleasant to listen to.
When instruments are competing with each other in a similar frequency range, cutting EQ can often be more effective than boosting. Sometimes you can eliminate unnecessary frequencies in signals to create space for other sounds to be heard more clearly without having to raise their volume. On an acoustic guitar for example, cutting the lower frequencies below 80Hz will do little to the perceived tone of the guitar if it's part of a full band mix, since it's the only the higher frequencies that cut through the mix anyway. Eliminating these lower frequencies in turn will leave more space for the more bass-oriented instruments in the sound spectrum.You'll be impressed with how much clearer your bass guitar part sounds when competing lower frequencies from the guitar signal have been removed.
The human ear is able to adjust well to the frequencies it hears, so after a long session, you're less likely to make accurate EQ adjustments that you'll be able to live with long-term. Give your ears a rest by taking frequent breaks when mixing. This will save you from having to go back to a mix to undo ill-informed EQ adjustments you may have made.
It's also a good idea to take advantage of an EQ On/Off button. Turning it off will bypass your EQ adjustments, which will give you perspective and let you know whether you're actually improving the sound of your mix.