The good thing about playing several different instruments is that you can grasp a better understanding of what music is and what it means. By playing several instruments you get different points of view, and discover things that you would have overlooked if you played just one of them.
After all this time playing saxophone I have learned just one thing that I can use to improve my guitar playing...but what a key thing: the importance of having a good tone. As any sax player knows, having a good tone is crucial if you want to get somewhere with your instrument. The quest for a good tone is for sax players something like the quest for a musical Holy Grail or some sort of philosopher's stone that can turn what you play from a mere string of notes into a meaningful musical expression.
For guitar players, getting a good tone usually means trying different effects (some reverb here, a bit of chorus there...), using strings of a different gauge, or simply turning the tone knob on their axe. That changes the "physical" qualities of the sound your instrument produces, but a good tone is much more than that. A good tone is not only something that makes your phrases sound good, but also makes you feel good inside when you listen to them.
My sax teacher used to put me in front of a wall with my sax and say: "Take a note and blow it until you feel how it goes out of the sax, bounces against the wall and then hits you. If you do not feel that, even if it sounds great, you do not have a good tone". I guess that not a lot of guitar players ever try something like that, but whatever the instrument you play, just try it. It is worth to spend sometime developing a good tone on your instrument. You will discover a new world of sound coming out of your guitar, piano, flute, banjo, or whatever. Believe me.
As with any other aspect of your playing, listening to other players is a must, but one should not limit himself just to those who play the same instrument. As a guitar player, my role model has always been Clapton, but I have many others, depending on which area I want to develop. For example, when it comes to improvisation, is not Clapton, but Parker, whom I follow, but I also like to listen to bass players, since their way of improvising is utterly different.
And when it comes to improve my tone, the choice for me it is obvious. Guess it? Maybe Santana with that amazing sustain? Nope. Coltrane with his powerful sound? Wrong. What about Albert Ayler and his sensual and organic way of playing sax? No (but this one is also a very good inspiration, I have to admit it) The answer is...Barbra Streisand. Surprised? Just listen to her and then imagine that you could do that with your guitar. Not even in your best dreams...