- keep practicing these pieces to death
That depends on whether your practice makes progress or not. Sometimes we need to learn how to let a piece rest and ferment for a while, working on something new. That said, you mention problems with HT, and I've got stuff for that.
Normally, I simply slow it down and make sure it feels comfortable at a manageable tempo, and then it speeds up naturally. Obviously that's not the case. This is a paraphrase of a technique I got from reading bernhard's posts on back messages:
Dropping notes:
This is only for passages that you truly cannot get Hands Together. It doesn't really work for cross-rhythms. If these are a problem, PM me as I have an article on them in Clavier Companion, but I left that at school by accident. Make sure you know both hands HS very well. Start by cycling the RH. Get it as smooth and desirable as possible. When you are happy, add the first note of the Left Hand, only the first. It may fall apart. That's okay, practice it until it's where you wanted. Then add the second note. Again, it may fall apart. That's okay. Repeat this process until you are playing the entire passage hands-together. After that, start with your Left Hand, and repeat the process with the hands switched. YOU MUST DO THAT LAST PART. Use this only for relatively small passages, as you can see it's quite time-consuming. I hope it helps.
Consistency is also a great thing. It's absolutely amazing how much improvement can be made if one practices a section every day. Not practices it to death, but makes sure, every day, that it's in top shape. The progress made this way is astounding. And, when there's no more, truly,
then it's time to drop it for a while, unless, of course, you have exams, as you do, and as I do.
- practice technique in isolation (scales, Czerny, Hanon)
That depends on what you mean by technique. Scales, Czerny, and Hanon are NOT technique in themselves, they are exercises. Technique is the combination of motions used to achieve the desired sound. So, what will you be doing in these exercises? I personally use them to focus on: playing from the knuckles instead of the fingertips, avoiding collapsing knuckles (when the joint almost bends backward), guiding the motion of the fingers with the wrists, guiding the entire mechanism with the arms and elbows, playing with the tip of the thumb and not the side, and, of course, relaxing as completely as necessary.
- pick up another (slow/ easy) piece to memorize and add to the list
If you find one you like, go ahead. Just make sure it doesn't get in the way of what you HAVE to do. In the future (and I don't know how you feel about your current pieces), try to get your teacher to assign you pieces you love, or make sure you find something to love in the pieces you're assigned before practicing.
- pick up the next assigned (tough) piece to memorize ahead of time
Same as above. If you like it, sure. If you don't yet, maybe you should burn a couple of recordings onto a CD first and let your ears get used to it, and eventually come to enjoy it. Also, unless your teacher requires it, take it to the lesson UNmemorized first. This way, your teacher gets a chance to point out any wrong notes and bad habits before you commit them to memory.
- work on sightreading
Always a plus, but don't consider it work. Sightreading is for when you need a break from actual hard practicing. You pick up an anthology of music, open up to a random page, and just play it. Sightreading improves with your ability to recognize patterns in a score, and to remember key signatures, time signatures, and clefs on the first go. You can also help yourself (in measures where the notation obscures the beat) by putting in vertical lines on the beats in each measure. Note any road maps, repeat signs, da capos and del segnos. Leave out all ornamentation. My keyboarding teacher also makes us break all ties, repeating the notes so that we have a clearer understanding of where the beat is at all times. There are loads of techniques you can learn. But part of sight-reading is simply becoming a better pianist and improving your technique and musicality, so don't forget those, either.
- [other]??
Listen to piano music. If you don't have one already, build yourself a classical piano library, and pop the CD's in every once in a while. It's enjoyable, it's relaxing, and hey, maybe you'll find one you really want to play, and you can start working towards it.
As to your time question, I haven't found a solution yet. I'd like to know too, as I'm auditioning for a performance degree in seven weeks, but I can't know. And perhaps it's better that way too. I can keep practicing as if I will make the cut, and even if I'm not up to par, perhaps the effort and improvement from my last jury will show the professors that I'm willing to put in the effort. Some things come fast, others come slow. It all depends on how your body is currently constructed, and what needs to grow/change to get you into form to play your compositions. Some muscles that normally only moved slowly with strength must learn to move with speed and agility. This can take months. Always keep in mind, though, that improvement happens away from the piano. At the instrument, you do everything you can to condition your body to play better the next day, not the current. You're teaching yourself to do something, and neither the brain nor the muscles learn without having time to rest and catalogue and reinforce the material to be learned.
Topic: need help with speed and HT practice