1 Silence please: Golf is a social game. It's one of the best things about it. But golf is also a game of concentration. Don't talk to a player (or anyone else - especially into a mobile phone) when someone else is hitting. Your voice is both a distraction as they swing - and an excuse if they hit a bad shot. You don't need to provide either of those.
2 Leave the course like you found it (or better): Fill your divots, fix your ball marks, rake your bunker. When your ball lands in an unfilled divot in the fairway or a footprint in a bunker, making your shot much tougher than it would have been, you'll understand why. On the green, fix your ballmark and perhaps one more.
3 Watch your shadow: Don't allow it to get in someone else's putting line or have it moving in the visual field as another player is hitting.
4 Watch your positioning: Do not stand too close or in the wrong area to another player. Directly behind a player's hitting/putting line can be a distraction. Really, anywhere that can encroach in a player's visual field as they swing. When on the green, if you want to "learn" from another player's putt, move to the correct line behind them AFTER they've hit the putt.
5 Sportsmanship trumps gamesmanship: You think you're gaining an advantage with the 'early walk' or the dropping of the bag or the 'cough' when they are putting? No, probably not. But you're definitely gaining a reputation as someone no one wants to tee it up with.
6 Tend the pin: It's never a bad thing to ask someone with a lengthy putt if they want you to 'tend the pin.' That is, keep the flag in the hole until the putt has been struck and then remove it before the putt gets to the hole area. Also, if someone is just off the green, ask if they'd like to keep the pin in or out.
7 Safety first: Know when to yell "Fore!" "Fore" is a warning that a wayward shot is coming. If you hit a shot that is headed towards another golfer, a course worker, any person - let them know a shot is coming their way. Seriously.
9 Return lost clubs: Players will inevitably leave clubs behind. Don't look at this as an opportunity to score a new wedge. You'll leave yours behind as well one day. This shouldn't even have to be said. But alas, it does.
9 Keep up with the group in front: Pace of play is all the rage right now. For good reason. I'm not talking about speed golf. Just don't fall too far behind the group in front of you and you'll be just fine.
10 Let faster groups play through: They'll be glad you did it, so will you. It's no fun for them to watch you plumb bob for a 7. It's no fun for you to know they are staring you down. Also, when you're playing through, be fast. Don't hunt for errant shots too long. Don't measure up every putt.
11 Don't laugh when someone misses a short putt: It's not funny to them. It won't be funny to you when you do it. And you will. (This actually goes for any bad shot)
12 Respect the game: Have fun, but don't be obnoxious. Screaming/yelling so that someone three fairways over can hear you...not cool. Cussing is bad - doing it loudly is dumb. Shake hands with each playing partner/opponent after the round. Take your hat off when you do it. Don't wear your hat indoors. And throwing your club/putter in disgust is never ever appropriate.
13 Putt 'em out: Unless your partner/opponent specifically tells you that your putt is "good" - don't assume it is and pick it up. We've all seen someone miss a three-footer and just because they are disgusted, pick the ball up or knock it away. The hole is not over until your ball is in the cup.
14 Understand buggy and trolley etiquette: Park in an area where you're not walking backwards after the hole to get back to your bag. Don't park where you're in the way of another player or another group. Don't start the buggy or electric trollley during another player's swing. Don't start the buggy as someone is starting to take a drink. Especially if they're wearing white.
15 Remember when you were a beginner: Don't be a jerk to those who aren't as skilled, or educated in golf norms. Rather than criticize, or get upset, offer to teach.
16 Don't (sandbag) inflate your handicap: If you're in a competition, whether an organized tournament or a friendly wager - don't inflate your handicap hoping to win a few quid or an acryllic plaque. The worst thing you can do in golf is cheat. It's the unforgivable sin. And this...is cheating.
17 Winner buys drinks after the round: If you won the big money (not condoning (or condemning friendly wagers)) - the least you can do is buy the group a round.
18 Nearest to the pin?: If you think your ball is nearest to the pin beating the existing pin marker, you should confirm your winning distance with your playing partners BEFORE REMOVING THE PIN MARKER to write your name on it.