Making room for every element in a busy arrangement is esential to a pro sounding mix.  Every instrument should be heard, if it doesn't need to be heard it shouldn't be there.  But most songs have instruments competing for "space" in the mix, covering the tone or accurate timbre of the instrument.  This is called "masking".  How can you avoid masking and have each instrument shine through without overpowering the mix? 

One technique I use is to think of each instrument in terms of two main frequencies.  The exact frequencies vary by mix, but the goal is to highlight different frequencies for each instrument.  A good example is the lead vocal and snare drum which often occupy similar frequencies and are usually panned center of the mix.  I may want to highlight the snare at 150hz and 3.5khz to bring out the body and snap.  The lead vocal would need to be different frequencies (complementary eq) so I may try to focus on 700hz and 2khz for vocal.  Of course this all depends on the context of the recorded tracks and what they need, but thinking about two main frequencies on each track is one way to keep the mix "in your head".

One key concept in making this successful is to use subtractive eq rather than always going in to boost these frequencies.  Boosting in general is more problematic than cutting since it raises levels (cutting into your headroom), adds comb-filtering and psychologically can fool you into thinking it sounds better just because it is louder.  In the example above, if you want to emphasize 150hz and 3.5hz on the snare the best approach may be a wide Q cut around 700hz.  This works even better given the emphasis you need on the vocal in this frequency.  On the vocal, cutting between 700hz and 2khz would put you around 1.2khz, which could impact the "intelligibility" of your vocals, so you want to be careful cutting in that range.  This is a narrow range as well so you may use a more narrow Q.  This may be a case where boosting would be better. 

Using High-Pass Filter (HPF) in tandem with the two frequency approach also helps you emphasize the lower of your two frequencies.  So back to the snare, a HPF around 80hz cleans up the bottom and puts a higher peak on your eq curve around that 150hz frequency you want to emphasize (without boosting it).

Besides emphasizing the two frequencies and cutting unnecessary lows, I often apply high frequency additions to some instruments and all vocals (8-14khz) to give more air to the mix.

Here are some general guideline frequencies that I often think about for various instruments in a rock setting:

Kick - 50hz-80hz, 4-8khz
Snare - 80-120hz, 2.5-4khz
Toms - 120hz-150hz, 4-5khz
Cymbals - N/A, I usually cut a lot of the bottom with HPF around 150hz and low cut shelving up to 800hz
Bass - 40hz-200hz (depends on the tone you are going for), 800hz-2khz
Guitar - 150hz-500hz, 1khz-3khz
Vocals - 200hz-800hz, 2khz-4.5khz