In most systems you learn a new kata every few months. It could be a single or couple of kata for the next grade, or learning a different kata for other reasons of your own. I would give the same advice to all students learning and practicing new kata — practice what you know. If you reach a certain point in a kata where you get stuck, practice what you do know. Whether you remember only the first three moves or all except the last three moves, or a couple of moves here and there with bits missing in-between, you should practice what you do know so you can ask about the other bits at the next class.
In most cases it doesn't take long to practice all the kata you know. If you do this then you should see a big improvement in your kata in a short time, which is what you would expect if you are doing kata every day instead of once or twice each week. But doesn't everyone practice almost every day rather than perhaps a last-minute brush-up just before you leave for class?
There are also different ways of practicing kata. There is the "quick run-through" to remind yourself of the pattern. There is "full-focus" as if you're being assessed for grading, or competing in the finals at tournament. These are the two most common methods I see practiced, but there are other ways too...
There is the "slow-motion full-focus" whereby you do your best effort and technique and stance all the way through a kata. Except everything is slowed and deliberate which gives you time to examine technique. The normal "block-block-turn-block-strike-block-turn" etc doesn't allow time to think about the technique you're executing nor the stance you're in. It especially doesn't allow for thinking ahead to what's coming up.
At a seminar I attended I was told "Jujutsu done correctly can be done slowly just as easily as done fast. The difference is that done slowly won't hide the mistakes." The same is true for kata.
There are myriad other ways of practicing as well - backwards (finish to start), mirror image, with a child attached to each leg (not by my choice ;-) ), blindfolded, with someone riding on your back... the list could go on. Which of these or others you might choose to do is up to you, just as it is up to you to determine what is practical and what is nonsensical.
Another thing to consider is why you're learning new kata. Is it for grading, competition or personal interest? In my opinion there is no point to learning a huge number of kata just to be able to boast about how many kata you know. And be cautious when you say you "know" any particular kata. It is more than just a performance routine - there is an incredible amount of depth to kata though few do more than just scratch the surface.
- Mick Todd