Another drill in addition to the turn & sprint drills previously described (e.g, with the coach throwing the ball over the fielder's head).
Have the coach hit fungos from HP over the fielder's head, with the fielders in their positions. This will help them learn to read the ball coming off the bat.
Remind the fielders of the flight tendencies of hits. E.g., an oppo field hit will tend to slice toward the foul line. Remind them to allow for the effects of wind. These things will help them take their initial drop step or crossover step in the proper direction and probably eliminate a head turn.
In the turn & sprint, be absolutely sure that they always turn their head - and never their body - as they go back.
Be sure their intial drop step or crossover step as well as their first few steps go directly back toward the fence. They don't want to make a big loopy turn as they head back. The big loopy turn will increase the chance of having to make an unnecessary turn on the ball hit on a line directly toward them.
Remind them to start back hard. They can always slow down, stop or even come back. But if they start too slow and the hit is out of reach, there is nothing to be done and there will be no catch.
On the previously described drills with the coach throwing the ball, ensure that they run through the catch. Throw the ball at the limit (or even a little beyond) of their range. So they learn not to slow down.
Sadly, very few players can properly execute the turn & sprint. Those that can, though, look like men amongst the boys.
Make sure to work this drill with IF'ers and OF'ers. Both need to be capable of executing the turn & sprint.