At the behest of my clarinet teacher and my violin teacher, I recently made two small purchases (<$20CDN in total) with unexpectedly huge results.
Violin: After being informed that my E string probably needed replacing, I replaced it with a different type -- A wound E, not the plain E that I had been using since my first lesson in early 2003. This one is a D'Addario designed to cut down on "whistling" in the high registers, and Vilma is quite enjoying it.
Clarinet: I finally got the go-ahead to advance to a harder reed, having played 2½'s from the beginning of that course of studies. Picked up two #3 Vandoren "56 rue Lepic" reeds to try them out.
Results: An immediate, audible improvement in sound and control on both instruments.
Listen to your teachers. Buy the best parts and supplies you can afford. And, while you're at it, get the action and intonation fine-tuned on those guitars, too. Music is intense enough without making things harder for yourself.