One of the most important lessons that I've learned is to make sure that I listen with my eyes. In a nutshell, audio, video and lighting technicians need to pay attention. Listening involves hearing, but it also involves seeing, in order to make sure that everyone and everything is heard well.
Whether it's something simple like making sure that the pastor's microphone is on when he starts to speak or something subtle like adjusting the sound in such a way that people can actually hear the guitar part, it's important that you listen by watching.
Next time you mix, look around. I'll bet you find that at least one person that's playing notes or singing a part that isn't being heard, and that he's doing something really cool that you'd like to hear.
Mixing well isn't just about knowing how to use the technology; it's about art, too. Part of creating good art is making sure that you paint with a full palette.