There is no more effective maintenance procedure than using keyboard memory and mental play. Make a habit of playing in your mind at every opportunity you have. The difference between a good memorizer and a poor memorizer is not so much "memory power" as mental attitude -- what do you do with your brain during your waking and sleeping hours? Good memorizers have developed a habit of continually cycling their memory at all times. Therefore, when you practice memorizing, you must also train your mind to constantly work with the memorization. For poor memorizers, this will require a lot of effort at first, but is not that difficult if practiced over an extended period of time (years). Once you learn mental play, this task will become much easier. Savants generally have problems of repetitive motions: their brains are cycling the same thing over and over again at all times. This can explain why they cannot perform many normal functions but can have incredible memories and amazing musical abilities, especially when we view these savants in the light of our above discussions about memory and playing music in your mind.
Maintenance time is a good time to revisit the score and check your accuracy, both for the individual notes and the expression marks. Since you used the same score to learn the piece, there is a good chance that if you made a mistake reading the score the first time, you will make the same mistake again later on, and never catch your mistake. One way around this problem is to listen to recordings. Any major difference between your playing and the recording will stand out as a jarring experience and is usually easy to catch.
Another maintenance chore is to make sure that you still remember it HS. This can become a real chore for major pieces, but is worth it, because you don't want to find out that you need it during a performance. Note that these HS maintenance sessions are not just for memory. This is the time to try new things, playing much faster than final speed, and generally cleaning up your technique. Extended HT playing often introduces timing and other unexpected errors and this is the time to fix them. Therefore, playing HS for both memory and technique enhancement is a very worthwhile endeavor. This is one of the best times to use a metronome to check the accuracy of the rhythm and timing, for both HS and HT play. The best preparation for recovery from flubs during a performance is HS practice and playing in your mind. Then, if you flub or have a blackout, you have many options for recovery, such as keep on playing with one hand, or first recovering one hand, and then adding the other. This recovery method works because flubs and blackouts rarely occur simultaneously in both hands -- it usually occurs in one but not the other, especially if you had practiced HS.
In summary, maintenance has the following components:
(1) Check with the music score or listen to recordings for accuracy of every note and expression.
(2) Make sure you can play the entire piece HS. You might practice HS very fast for polishing the technique.
(3) Practice starting at arbitrary places in the piece. This is an excellent way to test the memory and your understanding of the structure of the composition. If your mental play is good, you should be able to play from any note in the composition, not just the beginning of phrases.
(4) See if you can play very slowly without flubs or memory lapses.
(5) Play "cold". It will greatly strengthen your performance capability.
(6) Play "in your mind", at least HS. If you start this from the beginning when you first learned the piece, and maintain it, this becomes surprisingly easy.