The one I remember the most was taught to me by Garry Shider when I asked that I be turned up in the side fill. He told me, "If you can hear yourself, you're wrong."
Muruga taught me to stay in the pocket and to not sing from my throat but from my heart.
Mark Whitecage taught me that if you're think about what you're playing you're not playing it.
G taught me not to get fancy but to maintain the groove.
Perry Robinson taught me how to play free - just play.
There have been many others but those are the ones that I remember most often.
What lessons have you learned?