Write in the range of the instruments. Go to string players while you are writing and ask what is playable and what is not.
Avoid Keys with a lot of flats, we hate that.
Learn what bowings are and what they do, its very complicated. Also learn about strokes and articulations, we have a million.
Learn all the italian words and use them, dont use english.
Write in keys that use open strings.
Bowings are really a big part of it, that and the effects we can do.
As Fraser says, check out the great quartets of Beethoven, Brahms and especially Bartok.
Dont write too high for viola, that will suck
The biggest mistake is thinking like a pianist etc. You have to learn to think like a string player. We think about the line and the phrase, we have a linear way of thinking. We dont do chords well, avoid double stops and ESPECIALLY octaves.
We have a huge dynamic range, more than you can imagine.
Learn the following words, you will need them:
Staccato
Spiccato
Marcato
Martele
Colegno
Pizziccato
Sul Tasto
Au Talon
Jete
Con Sordino
Senza Sordino
subito
Rubato
Make sure you put in RESTS. We need a lil breather, we cant play constantly.
Know the difference between down bow and up bow, what they do and how to notate that.
You owe me 50 bucks for all this

Good luck with the quartet.
