There are a hundred things you can try to do to fix your slice, but if you don't understand one simple fact, the banana ball will continue to haunt your rounds. Here it is, from Top 100 teacher John Dunigan (@JohnDuniganGolf):
Your left hand controls the angle of the face at every step of your swing. So if the goal is to square up the clubface at impact, why would you flail it open on your backswing by cupping your left wrist? The answer is, you shouldn't!
Yet this is exactly what recreational golfers do as they swing to the top. The bad news is that once your left wrist cups and the face opens, it's difficult to return the face to square. Instead, you're forced to stop your swing just prior to contact and flip your hands at the ball. Combine this with the weak grip that most weekend players use, and it's goodbye power, hello spray.
Pros do just the opposite. First, most use a strong grip (left hand rotated to the right, like the photo below).
Then, when they take the club back, they keep their left wrist flat. Some even bow it (can you say DJ?). This keeps the clubface square all the way up, setting up a thought-free downswing where all you have to do is turn and then unload at impact. It's that simple.