I agree with practicing with plastic and hitting your mark. The main problem you have when practicing on a THS is the massive amounts of dry outside 10. You have to find a way to work on playing straighter and using plastic up 5, then up 6, and continue to move in to 10. Then work on smaller swings like 6-5, 7-5, and so forth. Everyone can move inside and swing the ball somewhat, but playing straighter isn't done anymore, and that is what is needed.
On spares, they are obviously important. Know in advance where you will stand and targt for every combination. Even write it down before you get there. You also must commit yourself to the plan and stick with it. Playing straight at spares is new to a lot of people and you have to trust it. With practice comes trust.
Have a plan of attack for the shot. Since you can't practice on the pattern, try to develop how you will start and try to maintain your plan as much as possible. Also have an idea how each ball you are taking works and what the expected progression would be. Preparation can be just as important as execution.
In conclusion, practice spares of course, work on angles with a plastic ball, have a good plan of attack when you start, and know equipment and how each ball works compared to the other.
On a side note, I agree with Dizzy about learning transitions and being prepared, but you can't expect transitions on a THS to come close to those on a flatter pattern.
