One of the most common reasons that pianists have problems with musical passages is bad fingering. Too many times I see pianists young and old glossing over fingerings in the early stages of learning, and then wonder why they have troubles getting a passage up to speed a few weeks later.
This posting will not be a treatise on what constitutes good fingering, which I consider a highly personal art (science?). However, here are some ideas on how to find the right fingers to play the right notes:
1. When you learn the notes and rhythms, learn the fingerings too. Playing a passage with correct fingering will lessen the amount of time spent learning the piece. The earlier you start, the more efficient the learning process will be.
2. When the printed fingerings don't cover all the notes you need in order to properly learn a passage, add them in pencil. This will take a small amount of elementary reasoning and a couple of tough decisions, but will pay dividends in the long run. Of course, it is somewhat redundant to have a finger number over every single note, but try to have the critical musical corners fingered as much as you need.
3. Once the passage is learned with the right fingerings, repeat with the exact same fingerings during practice. Repetition with the right hand shapes and finger crossings can make the process of working up a piece a relatively quick one. On the contrary, use sloppy fingerings and you'll have work to undo later on (see #5).
4. If you need to change a fingering, realize that you are giving up something in order to get something else. Take repeated single notes, for example. Most editions print changing fingerings. However, not all pianists agree. Deciding if you are a finger repeater or finger changer can be a matter of taste--just realize what each fingering type accomplishes and listen for the sound you want.
5. The longer you wait to change fingerings, the longer it takes to undo. Correcting mistakes that go back to the initial learning process take an awful amount of time, practice time that could be spent in other productive ways. There may be moments when you realize that the original fingerings you used no longer make musical sense. If you need to make this decision, remember #4--giving up something to get something else is fine, as long as it is for an ultimately musical reason.
So take out those sharpened pencils and get to work fingering your way to pianistic excellence without the hassle of relearning.
Next: Interludes: Some Thoughts on Teaching and Learning