The cardinal sin many players commit, even when they're good at the game, is playing too fast. It's easy when playing in a tournament to just hammer away on the ball and keep churning out shots, but you want to make sure that there is some thought behind what you're doing, especially on Tournament difficulty where one mistake can send everything awry.
Be sure to use your caddie advice on difficult approaches and putts, as a lack of putt previews and after-touch can really affect certain players' ability to post low scores. Also, don't be afraid to try some practice swings while chipping or putting just so that you can see how much power you might get on a shot.
Another mistake that can happen, especially if you're talking to people while playing in a tournament, is that you might forget to adjust the right stick before a shot, affecting where the club strikes the ball. This can be devastating for shots from the rough or sand, and it almost always results in a driven shot that will sail the green.
What do you do to ensure you play at a controlled pace?