In addition to what John said, generally having good form and a good release helps to avoid clipping the nose. Creeping can cause the string to move left on release, as can consciously opening the fingers rather than relaxing them. Someone sent a video in here recently where you could see him moving his head on release so his nose moved into the string. So: still head, maintain back tension through followthrough, let your fingers relax so the string can push them aside.
Sometimes I have had string hitting my nose that I couldn't diagnose, and just by working on my form it went away without my really knowing exactly what caused it.