After she gave a concert, Isreali pianist Orli Shaham gave me a great piece of advice - if you can't seem to be able to tackle a difficult passage, change the fingering. About 95% of the time for me, incorrect fingering is what makes a particular section impossible to consistantly play correctly.
In cases where I conciously use a particular fingering, I always write it down in the music. This way, if I later regress into a different fingering (which can happen without you realizing it), you can go back and find the better way quickly.
Another thing I suggest is to use the score whenever you practice, no matter how well you know a piece. Memorizing a piece is no problem, but more often than not, you will look at the music to a piece you've known for months and spot something you missed the first time around. Inner voices and harmonies, subtle dynamic changes, phrasing, and tenutos are just a few of the things which are easy to overlook if you never use the music once you've memorized a piece.
I use a timer and set a goal for myself every day in order to practice. Usually I don't need this sort of thing, but everyone has lousy practice days. On these days, setting a time goal for yourself will ensure that no matter how lousy things seem to be going, you will still work enough for your own satisfaction. In these cases, practice as slowly as possible - rushing will usually make things collapse when you're not in the mood to practice.
Piano is not work. This advice is probably moot on this forum, but many children and adults perceive piano practice as a chore. All it takes is a mental effort in the contrary direction, and suddenly practicing becomes something to look forward to.
Despite what's been said, slow practice always helps with playing faster. Physically, it's easy to hurry a dense stream of notes without missing any, but if you only practice at top speed, you will lose articulation. Slow practice will ensure that you hit every note precisely. Anybody can play fast, but many less can play beautifully.

My last bit of advice - apples are tasty and give you energy. Eat them before practice, it makes a world of difference.

-Motrax