Recognize that equalizing when mastering is much different than equalizing when mixing. You can no longer single out individual instruments as you can in mixing. Every adjustment you make with an equalizer during mastering will affect the entire song. For instance, if you add low end to beef up the kick drum, you will also be enhancing the low end of the bass guitar.
Make subtle adjustments when equalizing a song during the mastering process. The goal is to improve the overall sound of the song, while keeping the adjustments sounding natural. In general, you can accomplish this by using a wide Q, or equalizer bandwidth, and making cuts and boosts of no more than a decibel or two. However, these rules are only guidelines. If an adjustment sounds good to your ears and achieves the desired effect, that is what matters.
Use a compressor in the mastering process to manipulate the overall dynamics of the song. A compressor is often used to contain the dynamics of a song so that its overall level can be increased. A compressor is also sometimes used to act as the "glue" that brings a song together. As with equalizing during the mastering process, a compressor should be used as transparently as possible. Most of the time, a ratio between 1.25-to-1 and 4-to-1 is desirable in the mastering process. The less gain reduction that is used, the more dynamic and transparent the song will sound. The more gain reduction that is used, the less dynamic and more compressed the song will sound.
Use a limiter in the mastering process to "limit" the audio peaks in the song and increase the overall volume of the song. By limiting, or stopping, peaks from going above the maximum output level, you can increase the overall level of the song. This step is important when it comes to bringing the volume of the song up to a commercial level. However, be careful not to apply too much limiting to the song, as this can cause the song to sound distorted. It can also cause the song to sound squashed, where the song sounds lifeless and you can audibly hear the peaks that are being limited.