I find myself in the same boat all the time.  But like the pros say, play it one shot at a time.

I'm guessing you are like me in that you give a lot off attention and focus to your difficult or important shots.  Such as going for the green in 2, or hitting that important drive to set you up with a great look at sticking the pin.  But what I find myself doing, is that once I hit that good shot, and I tend to lose focus on the next shot just assuming that the last shot I hit was so awesome that I can just close my eyes on this easy shot and everything will be fine and dandy.  And that's where you throw away shots.  I tend to play well in tournaments because I put a lot more focus into every shot. 

Try to give the same amount of attention to your difficult shots as you do your “easy” shots.  When you're 80 yards out, make pretend the green is smaller than it really is and really focus on trying to knock it in there tight.  When you have a short chip, make pretend that you need to get up and down to win a $10 bet.  Doing so may not make golf as enjoyable, as your going to get wound up and stressed out…but that's kind of what it takes to play really good golf.  You can't just expect to go out there, find your ball, pick a club, take one practice swing and everything is all good.  The game just isn't that simple.