Compression is a subtle effect that evens out the dynamics of an audio track. The spikes and peaks are lowered, and the quieter sounds are boosted. As a result, the overall volume of the track can be pushed higher without distorting, as the level is more predictable. Compression is essential for preparing the individual channels for a mix. When mixing down, use a compressor very sparingly on the master channel. Over-compressing a final mixdown can destroy a lot of the dynamic shifts in an arrangement, making it sound dull and flat.
Equalization is another vital component in audio mixing. Boosting on cutting frequencies within a certain sound gives you the ability to sort each sound into its own space within the frequency band. You prevent the similar sounds from bleeding together, allowing you to hear all the elements in the final mix. Again, when mixing down, the use of EQ should be fairly subtle, so as not to thin out the mix, and as a rule of thumb, more frequencies should be taken away than added. Turn down unwanted frequencies, rather than boosting the important ones; too much of a good thing can make a final mix sound woolly and crowded.
Practically all professional recordings are produced in stereo, if not 5.1 or some other more complex spatial medium. Two-channel audio is designed to add a sense of space and depth, and multi-channel mixers always include pan controls on each channel. Make sure every channel is panned to the right space before commencing mixdown. Keep the bass, snare and kicks in the center to tie everything together, and move peripheral sound "garnishes" out to either side to broaden the mix. Cymbals, effects and pads benefit particularly from this. Don't pan anything right out to the extremes straight away; start small and gradually increase. Also, try and keep an equal volume of elements in the left and right channels. Listening on headphones can help you determine if you have too much happening in one speaker.
Before finalizing your mastered mixdown, play back your track through a different pair of speakers, listen on headphones, transfer it to your cellphone and listen to it through the built-in speaker. Listen to your track through as many different sources as possible before cementing your mix. Once you start to give your track out, you can't control what kind of device it will be played on. It may sound great on your powerful studio monitors, but if the distribution of frequencies relies on them to sound good, it may sound terrible by the time it reaches your audience. You may need to make compromises to get it sounding good on weaker speakers, but it is worth it not to alienate certain listeners.