Start practicing a piece or part of a piece at the end instead of the beginning.
1. Decide how many notes you can handle at one time. This could be anything from less than a measure to a whole section of the piece: it depends on how difficult the passage seems to you. Mark each fragment with a bracket.
2. Number each bracket, beginning at the end.
3. Write a finger number over the first note of each fragment. Make sure this fits the fingering in the previous measure. Otherwise you may waste time by practicing with fingering that will have to be changed later.
4. Play bracket #1 perfectly four times.
5. Play bracket #2 perfectly four times.
6. Play brackets #2 and #1 together until they feel easy.
7. Continue working backwards, adding one fragment at a time until you reach the beginning of the challenge passage and can play the whole thing smoothly from beginning to end. Then repeat it several times for reinforcement.
8. Start several measures before the difficult passage, and play through to the end. When perfect, repeat four more times.