Some turnovers are created by the defense out playing the offense. Some turnovers are created by errors of commission (nobody's perfect). I think most turnovers are created by bad decision making. Bad decisions are made not only by players, but also by coaches. Coaches can prevent turnovers by teaching decision making in practice, controlling who handles the ball and by making their players aware of the situations that create turnovers.
The ball handler passing the ball to a player in a situation he can't handle creates many turnovers. For example, throwing the ball to your 5-8point guard in the lane that is protected by 3 defenders 6-7 or taller. That is a turnover waiting to happen. How about coming down on the break, you're in the middle and you have Shaquille O'Neal on one wing, Tracy McGrady on the other wing. You decide to give the ball up 25 feet from the basket. Throw the ball to McGrady; it's probably a basket. Throw the ball to Shaq, probably a turnover. Throwing the ball to a player who is not used to having to handle the ball in an open court situation creates turnovers. As a coach, you must teach players to consider other's abilities when making a decision. Don't put your teammate in a position he can't handle.
Control Who Handles The Ball
The ball comes off the backboard and your 6-8 power forward comes down with the ball. He knows the team wants to fastbreak so he looks up the floor, put the ball on the floor and decides to lead the break. Turnover waiting to happen (unless of course he is a player that should be leading the break). Players don't look at the big picture. He feels he can dribble up the floor to get a quick trigger for the break. Maybe he can. But, how is his passing on the run, how is his decision making? There is more involved with leading the break then just dribbling the ball up the floor. That is a situation that has to be taught in practice. The ball has to go to a ballhandler, even if it means delaying the break for a count. Getting up the floor a step slower is better than turning it over.
Construct an Appropriate Offense
A coach goes to a coach's clinic and sees an offense he likes. He comes back to his team and installs the offense. He has considered who the shooters are and who gets it in the post, but has he considered the personnel between the entry and the finish. Did he notice that his non-ballhandling forward has to come away from the basket to reverse the ball? Can the player handle that job? Sometimes coaches create turnovers by putting their players in positions that they are not suited for. You might possibly have to run an alternate offense if you have players in the game that can't handle the responsibilities of you main offense.
Teach your player that there are areas where they should not handle the ball. Don't bring the ball to the corners, don't stop your dribble at half court, especially near the sideline. These are situations that invite turnovers.
Teach The Rules
It is amazing how many players don't know the rules and the effect that they have on the game. For example, your position on the court is determined by where your feet leave the floor. For example, the ball is in the frontcourt and player A passes to player B, who has jumped and landed in the frontcourt. Turnover! Why? Because technically player B is in the backcourt when he caught the ball because he has not established positioning the frontcourt due to the fact that he was still in the air when he catches the ball. Backcourt violation!
Of course there are many other situations that create turnovers, such as one-hand passes, cross-court passes, turning you back on the play, etc. Coaches usually have a pretty good handle on those situations, even though it may take the players a little while to catch on. It is the other subtleties, such as who handles the ball and where, that coaches can really affect and thereby cut their team's turnovers.