Definitely keep your head up, not that pro hockey players seem to do it anymore. Can't stand how many arguments I get into over players being concussed because people never blame the hittee for putting themselves in such a situation, just the hitter for not letting up, as if that's how you play defense.
If I could only teach one thing, it would probably be to never give up on a play. Back check that guy on the breakaway even if you know you're not going to catch him because you can collect a rebound if it happens instead of giving them a second chance. Especially when it comes to kids leagues or beer leagues, people make mistakes. Assume they're going to make a mistake and back check so you can be there when they miss the puck. If you're a winger in your zone and the other team has the puck behind your goal line, pay attention to the point man you're covering instead of just floating around, they'll sneak in. I've only played beer league because my parents couldn't afford hockey for me as a kid and it drives me nuts how lazy players can be. It's not surprising when I'm the only forward on my team that actually backchecks.