You must examine the causes of the problem. What is causing the inaccuracy?
1. Not pressing the keys right in the middle?
2. Using the fingers to reach for the keys instead of using the arms to position the fingers?
3. Using the arms to position the hands, but since the tiny arm movements result in large movements at the level of the hand they need to be fine tuned?
4. Not knowing the notes?
5. Awkward co-ordinations due to awkward note sequences?(for instance, a descending passage that has ascending notes inserted so that the fingers keep going down automatically when they should go up)?
6. Hand span (difficult to play tenths accurately if your hand is small)
7. Using inappropriate fingerings/movement sequences that make it almost sure that you will miss the correct notes?
8. Other?
As you can see, it is difficult to prescribe a general remedy that will be effective for every possible situation.
I found that most of what has already been said will work for some kinds of inaccuracy, while not working for others. So try them all and if it is not working try something else.
I will add two more – a general attitude, and a specific practice procedure:
1. Make accuracy the aim of your practice. Forget about rhythm, forget about speed, forget about musicality. Have a practice session in which you will completely concentrate in being accurate. Do not leave the piano until you not only get it right, but cannot possibly get it wrong. If you are pressed for time work on small sections. If you have the whole day, work on the whole piece.
2. If you have a digital piano, switch it off, and play without sound. This works wonders for accuracy (and also memory – since you cannot use sound to guide you anymore: you must know the notes/keys). Without sound to distract you, you will be able to play attention to finger placement in the keys, movement patterns, fingering and so on. Needless to say, this is to complement other practice strategies, not a substitute for them.
I hope this helps,
Best wishes,
Bernhard.