Wow, this is a loaded topic.  So who here has issues with having some idea of where the ball is going before you hit it?  I know I have had, and still have, issues with this.  Everybody does, even the pros.  There are so many personal ways to control the flight of the golf ball, as evidenced by the great number of classic players who all had different swings.  There's no one correct way to do it, at least in my book.  But if you are struggling with that slice, hook, or whatever, here are some basic things to know about what the club does to the ball's flight, no matter what you swing like.
 
The way I see it, there are two major elements that affect the flight of the golf ball--now, I'm talking about directionality, rather than trajectory.  I'll talk about high and low shots in another blog. 
      Anyway, the first element is the direction the club is traveling relative to the target line at impact.  And there's only three possibilities here:  The club can be going directly down the target line (straight), to the right of the target line (inside out), or to the left of the target line (outside in).  This direction largely determines the ball's starting path.
      The second element is the angle of the face relative to the club's path of travel.  Notice I didn't say relative to the target line.  Again, there are only three possibilities:  The clubface can be square to the swing line, or open to it, or closed to it.  The clubface angle determines the curvature of the golf ball's flight.
 
So check this out.  I'm going to list all the possibilities and the basic ball flights they produce.  Now this is not set in stone--there might always be exceptions--but I think this is a good basic guide for diagnosing your ball's flight.  And knowing what's happening at the point of impact is a really big benefit when you're trying to work on something.  The formula will go thusly:
 
Swing Path Relative to Target Line + Clubface Angle Relative to Swing Path = X Shot Shape.
 
So here goes:
 
Straight + Square = Straight
Straight + Open = Fade/Slice
Straight + Closed = Draw/Hook
 
Inside Out + Square = Push
Inside Out + Open = Push Fade/Slice
Inside Out + Closed = Push Draw/Hook
 
Outside In + Square = Pull
Outside In + Open = Pull Fade/Slice
Outside In + Closed = Pull Draw/Hook
 
So do any of those ball flights fit you consistently?  Then that's probably what your club is doing at impact.  That's the first step to figuring out how you want to go about controlling your ball's flight.
 
I'll be the first to tell you that I am not a qualified swing coach, and I do not posess the greatest swing.  Therefore I won't try to offer any tips on how to fix your swing path, because I don't want to take responsibility for leading people astray.  But it is pretty commonly agreed upon that the best way to strike a golf ball is with a swing that is either right on line with the target line, or very slightly inside-out.  Some people have made outside-in swings work, but not many.  Anyway, to accomplish striking the ball from the inside, the only thing I can confidently tell you is that as long as your body is lined up properly--and by that I mean your shoulders, hips, and feet (not just your feet alone)--then if you remember to start the downswing with your lower body rather than your upper body, then you will go a long way towards getting the club on the right path.
 
However, let's talk about this clubface angle at impact.  This, I believe, is what most folks struggle with.  And most people struggle with a clubface that is too open.  I think many golfers do know that they are supposed to initiate the downswing with the lower body, but they exaggerate that move with a very aggressive and quick turning of the hips (hint: this happens often when a golfer really wants to crush one).  What happens when you do this?  Well, when you get your lower body that far ahead of your hands, the club "gets stuck" (heard that term before on telecasts?) behind you.  If you could freeze-frame close to the point of impact, you would see that your hands are way too far ahead of the club for way too long, and this makes it near impossible for you to release the club in time to square the face by the time it hits the ball.  The result?  A weak fade or slice.  You lose a lot of distance because although you've retained some lag by keeping your hands ahead of the club, you never get a chance to fully release it.  This is Tiger's big miss, because he has such powerful and fast hips, and he often can't keep them in check, especially when he's hitting a driver.
 
So how do you fix this problem?  Well, you do need to start the swing with the lower body, but you need to tone it down a notch or two.  Make a smoother and slower transition...you might need to slow your whole swing down.  But this will give your hands more time to get out in front of you, allowing you more time to square the face at impact.
 
You might suffer from the opposite problem--face too closed at impact.  Are you hitting wild hooks?  You might actually need to speed up your hips slightly, because the club has now gotten too far in front of you, and you've released it before you've even gotten it to the golf ball.  So try getting a little more aggressive with your hip turn and see if that gets the club behind you a little longer.  Again, don't make it a jarring action...it still needs to be smooth, just slightly faster than before.  There is so much timing involved with this game!
 
I'm going to stop here because there is really way too much that can be said on this subject.  Just go with this basic knowledge and see if you can diagnose and fix your problem next time you hit the range...or fine-tune a certain shot shape you like to hit.
 
One final note:  While it would be ideal to have one of the ball flights that start out straight and either fade or draw slightly, that might not necessarily be your most consistent shot...so experiment a little if you need to, and settle on the shot that fits your eye and your game.  For instance, I hit a slight push-fade about 90% of the time.  It's not a big push to the right, but it's still a push.  For a standard 7 iron shot, I might push the ball about 3 yards and fade it another 3 to 5.  Lee Trevino used to play this way.  This shot might not give me the most distance, but I like it because I know I almost never miss left.  That's a nice feeling!  You might like this shot, or one of the others.  Like I said, find the one that fits your eye, because confidence and belief in your shot is just as important as having proper swing mechanics.
 
Hope this helps!  Let me know if you have any questions.