I've posted already about setups to hit the fade and draw and the D-Plane (which explains the ball flight laws). My big fall flight error (like most amateur golfers) is the slice. Sometimes I set up to hit a draw and still end up hitting a slight fade (works out okay sometimes), or sometimes I'll set up to hit a fade and it will become a slice. That tells me that I'm compounding the left-to-right ball flight with my setup. And I've done the same thing with the draw too--setup to hit a draw and then end up hooking the ball off the course. Unfortunately, there are times when I've set up to hit one and hit the opposite (I'd rather not talk about those times right now).
Recently I was hitting fairly straight tee shots with a slight fade and was trying to figure out why I couldn't hit the tight draw that I was hitting a few days before. Everything SEEMED the same, but it obviously wasn't. Then I hit on it: I was NOT turning my shoulders back fully to 90 degrees. I noticed that my left shoulder pointed in front of the ball at the top of my backswing, which was giving me a slight outside-in swing path. That path along with a square club face should produce a fade; if the face was open, it would produce a slice (basically a severe fade from which it's hard to recover). Anything used on top of that to exacerbate the outside-in swing path would produce a more severe fade or slice, even with a relatively square club face.
Golf is really about trying to diagnose and repair what's screwed up during a round, because it's easy to get lazy and let bad habits or old tendencies creep in without realizing it (e.g., partial shoulder turn, right leg straightening to much, not keeping the arms in front, etc.). If you're getting a slice, it can be a lot of things, but I know I have a tendency to get lazy with the shoulder turn (which can cause a laid-off position at the top). I have a tendency to let the arms get too much across my chest (too inside leading to outside-in), and I have a tendency to get too handsy with my takeaway (too inside on the backswing that can lead to OTT).
The reason it's tough to diagnose why one might be fading, slicing, hooking, pulling, pushing, or drawing when it's not intended is that ball flights are a marriage between swing path and face at impact. If you KNOW your path is inside-out but you're push-slicing then you have an open club face. I believe this was more my issue when I first started golf--I simply was blocking the club face open and would often end up with a push-slice. Sometimes (unless you're really paying attention to yourself compared against the fundamentals), you may have trouble diagnosing what you're doing wrong.
Given that I noticed (this time) that I wasn't getting my back to the target at the top, I hit on the idea of using a variable shoulder turn to try to hit cuts, straight, and draws, especially since the traditional setup ideas to hit them usually give me severe slices and hooks instead of the intended flights. What if I simply used less shoulder turn for a fade and more for a draw? Then I could aim left of my intended target and hit the fade and vice versa for the draw. I wouldn't have to worry about all the other aspects of hitting fades and draws. If you think about it, it's the equivalent of opening or closing the stance to hit a fade or draw, but adjusting the stance requires the same full turn on both.
Here's something to try: To hit a fade, try turning the shoulders so that the left shoulder points in front of the ball at the top (from your perspective looking down); you shouldn't be able to see your left foot because your left shoulder will be shielding it from view. For a draw, turn more fully so that the left shoulder points behind the ball and covers the right foot from view. A straighter ball will be somewhere in between. Don't do anything else differently (except for where you aim)…use the same grip, square stance, takeaway, and ball position. The point here is to manipulate only one variable: swing path (and to do so as simply as possible).