It's all about frequency seperation! I was going to try and make some 'clever' analogy between trees..and space...and crowding...it all made a lot more sense in my head.
The 'vanishing guitar' syndrome is almost always because your guitar is trying to occupy the same range of frequencies as one or more other instruments. The top end might be clashing with the cymbals, or the bottom end might be clashing with the bass or kicks. The narrower you can get the band of frequencies the instrument lives in (without sacrificing the tone) the tighter your mix will sound. There aren't any concrete rules as to exact frequencies, as much of it is down to your own taste and the kind of sound you want. But a rough example might be:
Imagine a typical 'swept' metal guitar sound (lots of bass and treble, not much mids). It might give the snare plenty of room to breathe, but once you add cymbals and bass to the mix, they'll start fighting for your attention
That's how I tend to look at my mixes anyway. I mean, I don't have exact cut off points for different instruments but if I add a really warm mid-rangey lead sound, the first thing I'll do is kill everything under 100hz with a high pass filter so any backing/rhythm guitars don't get overpowered. Stuff like that. You'll always get a little crossover. Some instruments naturally occupy a wide range of frequencies. But in those cases it's just about careful arrangement and composition.
Also bear in mind that overtones and harmonics and other things will increase the space in a mix an instrument occupies. The body of my bass tone might be in the lower mids, but there's also some additional emphasis placed on the high mids to bring out pick attack. Again, it's just a case of careful planning and writing; knowing when to ease up on the cymbal action, or when one instrument isn't complementing the others.
Other than all of that...best thing to do is find mixes you like, and really train your ear to key into where the engineer has put emphasis within it. By doing that you'll also learn to zero in on offensive frequencies that you can kill altogether within a tone.
I'm waffling now.
Hope it helps even a little! Sorry if I've stated the obvious!