How can I practice composing for an orchestra? Should I take Beethoven's Fifth Symphony, analyze and then copy everything down on manuscript paper?
I would like to hear some techniques for practicing orchestration.
Good question; this is a much harder thing to practice than, say, writing a modulation to the dominant.
The most efficient method will be through score study. Study scores of other composers and see how they orchestrated. What do they do that is successful? What do they do that isn't successful? How are instruments treated in relation to other instruments, and how does this change over the course of music history?
As an example: a lot of composers use the tuba as just another string bass. Other composers use the tuba as another trombone. Sibelius often used it as an extra French horn (!). Still others used it as a solo instrument. Getting a sense of how various instruments are used, and how dense the rest of the orchestra is in each case, and the range for each use (and so on and so forth) is paramount.
I wouldn't worry about re-writing other scores; just seeing the scores and listening with recordings will be enough to give you an idea of balance, timbre, etc. And make sure you listen to multiple recordings of the same piece; just because an orchestration doesn't work in one recording doesn't make it a bad orchestration. It could just be a bad performance!
Famous orchestrators include Ravel (see especially his orchestration of Mussorgsky's Pictures at an Exhibition), Berlioz, Rimsky-Korsakov, Richard Strauss, Respighi, and Mahler. But they tended to orchestrate for full orchestra; you'll want to start with smaller chamber groups to get a sense of instrument range, role, and timbre first.
While score study is efficient, it doesn't give you an opportunity to test out your own ideas. If you have an ensemble (or various chamber groups) at your disposal, try arranging music (or composing your own) for these ensembles. Then, when you have these groups perform your music, you can hear first-hand what your orchestration sounds like. This is the best way to hear how your ideas work (and sometimes don't work).
Even better is if you can then make revisions to your arrangements and see how the new orchestration compares to the prior one(s).
It's also helpful to write some solo works for instruments to really learn about each instrument's range, how its sound quality changes, and what special techniques it has. When you're composing for orchestra, it can be tough to force yourself to think about composing for a single instrument; but this awareness of each instrument's capabilities is vital to any successful larger orchestration.
Lastly, you can try reading books on it. Samuel Adler's The Study of Orchestration is the de facto standard, and it comes with a workbook to work on exercises. Kennan has The Technique of Orchestration (which also has a workbook), Blatter has Instrumentation and Orchestration, and Piston has Orchestration. I've also recently encountered How Ravel Orchestrated, which seemed very promising.