Both Silva Luna and CT64 have it covered well - thorough study on each instrument as well as harmony, orchestration and I'll throw counterpoint into the mix as well. As far as text references go, Rimsky-Korsakov's book Orchestration and Berlioz's Treatise on Instrumentation still have much valuable information to offer despite their age, because many orchestral instruments have not changed that much since their days (perhaps the way they're played has changed though). Walter Piston's Orchestration book is also a highly used resource in my meagre collection.
However, the most useful thing (speaking personally of course) has been putting in years of study and especially detailed analysis of other composer's scores. It has nothing to do with copying what they do, but learning from how they do/did it, and deciding whether similar techniques will work for you in your approach.
There's a lot more than just knowing how high or low an instrument is capable of playing, there's also whether you're writing in the most effective range for that particular instrument with the subject material you're presenting, will it balance well with any other instruments playing at the same time, whether it's capable of the type of articulation you've requested, can it effectively play that fast, etc, etc, etc, etc. There are a great many variables, especially in an ensemble the size of an orchestra.
Among my favourite orchestrations are Beethoven's 3rd, 7th and 9th symphonies Berlioz's Te Deum and the Symphonie Fantastique, Holst's The Planets, Respighi's Pines of Rome, Bartok's Concerto for Orchestra, Mahler's 1st Symphony and plenty of other concert favourites - there's a reason why they're popular with orchestras.