I'm sure many of these have been mentioned before, but...
I always find that watching the other persons ball off the tee really can lower frustration later in the hole when you're driving around looking. Watch from behind the ball, so you can see the entire flight. We hardly ever loose golf balls this way, even in the shallow woods and rough.
When the other player is taking his or her shot, don't figit. Don't make noise, just stand or sit there quietly.
Repair tool on the green.
No cell phone.
First to putt puts pin back in, sometimes I even hand the other player his wedge or putter cover if nearby (if only 2 players).
Park the cart correctly such to not be an obstruction in the way or be within the players sight.
For chips and really any shot, I try to get out of the player's vision completely.
Help look for a players ball.
If a long putt, ask if they want you to pull the pin or just take it out.
When going to the green, take ALL of your clubs that you will need (wedge or two, and your putter).
It's a gentleman's game, so I always give praise to a good shot or drive. I don't play it competitively, except against myself.
Probably most importantly, I try not to do anything to slow down the game for my partner(s). I think the worst thing in golf is when you feel rushed because others are breathing down your neck. The worst. I shoot terrible and the game/sport just loses all interest for me.