Compose With Orchestration in Mind
When you’re writing music, at least have a basic idea about the orchestration while doing so. Writing something completely disconnected from the orchestration will cause compromises in the orchestration process later.
The problem that will arise is that when you for example try to orchestrate a melodic line later on, you will want to pick the instrument(s) for this line that “struggle” the least when playing it or you might put it in instruments that can somehow pull it off but don’t really have this line sitting in an optimal way on their instrument. Knowing what instrument you write for will also influence your writing as you might be putting more effort in staying in a proper range with the melody or in general writing more idiomatic for the instrument desired.
A good example what happens if you don’t think in orchestration when composing are the many rather unattractive orchestrations of pieces that have originally been written on the piano. The typical left hand arpeggios which sound great on the piano are very tricky translateable to orchestra without sounding silly. Most of the time, to get a decent result you need to go one step further and not only orchestrate but re-arrange the piece, changing these figures into something more orchestra-friendly which in return will cause the structure of the original piece to start dissolving, so you start fixing this etc. etc. All these things can be very easily avoided by just thinking more in terms of orchestration when you’re writing.