Etiquette Part 2:

On the green, remember:
-Don't step on your fellow players putting lines -- the imaginary line that connects the ball to the hole.
-If your ball is on a player's line, volunteer to mark the ball.
-If you're ball is not furthest from the cup:
-Mark your ball, either with a plastic marker or a small, thin, dark coin such as an old penny.
-After you have marked your ball, place your putter down at a 90-degree angle with the heel touching your marker.
-Move the marker from the heel to the toe of your putter. Reverse the procedure to return the ball to its original position.
-Do not stand where you might distract a fellow player and don't move.
-Don't make any noise when your fellow player is preparing to putt.
-If you don't have a caddie and are asked to tend the flagstick, make sure you aren't standing on anyone's line.
-Hold the flagstick at arm's length so the flag doesn't flutter in the breeze, and make sure your shadow doesn't fall across the hole or line. Loosen the bottom of the flagstick so it doesn't stick when you try and remove it by pulling it straight up after the other player has putted. The flagstick should be removed right after the player has hit the ball.
-If you lay down the flagstick, lay it off the green to prevent doing any damage to the green.
-Generally, the player closest to the hole will tend the flagstick.
-After everyone has putted out, immediately walk to the next tee.
Miscellaneous:
-If you hit a tee shot into the woods and suspect that it might be either lost or out-of-bounds, the Rules of Golf allow you to play a second or provisional ball.
-You then have five minutes from the time you reach the spot where you suspect the ball landed to find the ball. If it is not found within that five-minute period, you must declare it lost and play your provisional ball with a one-stroke penalty
-If, however, you play the provisional ball and subsequently find your original ball in-bounds, you must pick up your provisional and continue to play the original ball, in-bounds.
-Out of bounds balls are assessed the penalty of "stroke and distance."
-For a complete discussion of the Rules of Golf, visit the USGA's web site.
-For safety's sake, never hit when there's a chance you might be able to reach the group ahead of you, and anytime you hit a shot that you think even has remote chance of hitting any other players, yell "fore" immediately, and make a point of apologizing to any players your ball lands near.
-Displays of frustration are one thing, but outbursts of temper are quite another. Yelling, screaming, throwing clubs or otherwise making a fool of yourself are unacceptable and, in some cases, dangerous to yourself and others.
-As a player, you also have a responsibility to learn and understand the Rules of Golf.
-Five of the most common Rules are those deal with Out of Bounds, Lost balls, Unplayable lies, cart paths and water hazards.
-Moving on, if your group stops for refreshments, either at the turn (the end of nine holes) or following the round, volunteer to pay for them, as well as offer to buy drinks for the caddies.
-Whoever pays at the turn, however, should not be expected to pay for refreshments at the end of the round.
-Finally, at the end of the round, shake hands with your fellow players, congratulate the winners, console the losers, and thank them for their company. At the end of the day, the great pleasure of the game is the time you get to spend with your friends whether old friends or new friends you just made through the game.

Source: http://www.pga.com/play/etiquette