When you get to a certain point in your golf game--that point being where you can hit a ball with reasonable accuracy and distance control (read: if you are a sub-90s shooter, usually)...you can start to think about the best place to leave your ball on a green for your putt.
 
This is much easier if you have a regular course that you play on.  If you haven't yet, go study the greens next time you go play.  Really look at all the major slopes on the green (little undulations don't necessarily count), look where the pin plugs are so you know where they might put the flag sometimes...and commit them to memory.  This is very important stuff, because when you know the slopes, you can really affect your score based on where you leave the ball on your second shot.
 
I follow  Ben Hogan's old method, that is, his goal was always to leave himself an 8 to 10 foot, straight uphill putt.  Why?  Uphill putts are easier, because there is little danger of screaming the ball by the hole and having a lengthy comeback putt.  This is why you ought to study the greens as much as you can when you play.  On most good courses, there are pin locations such that you WILL 3-putt if you leave your ball in the wrong spot on the green...and there are also spots on the green relative to that pin that will give you a reasonable one-putt chance.
 
All this affects your club selection when you go to hit the shot.  How about this...say a green has a dominant slope down to the fairway.  The pin is right in the middle of the green, and it's 150 yards away.  I obviously don't want to go past the hole, because then I get a nasty downhill putt.  And sure, we all love to stick one close, and maybe it's a 7-iron to get close to that center pin...but if I don't stick it right next to the flag I might have some kind of tricky sidehill putt that I have little chance of making.  So I would use an 8 iron or choke up on that 7 iron, and intentionally leave my ball in front of the pin...now I have a nice, uphill putt.  I have a reasonable chance of making it, and I probably won't three-putt because it's easier to stop the ball close on an uphill putt as compared to a downhill putt.
 
Try to leave more uphill putts next time you go play.  Your putting nerves will thank you for it!