I might get from flack with this one:
With new virtual instrument products coming out faster than ever, creating mock-ups have become easier and easier. Though, they have also created some bad habits with composers.
My one tip that has been said ad nauseum, but must be said again is: write every note in your mock-up. If you have a harp glissando and there is a great keyswitch with it pre-written, nix it. Write each note in that glissando. Experiment with different variations of notes. Different movements. You will achieve much more rich and original sounds, and a better understanding of the instrument. And that is only for the harp!
I can't emphasize this enough, NEVER write using an ensemble patch. Feel free to jam on it, trying to discover your melody and harmonies. But never actually write with it.
Write each line with each instrument. One line for Violins 1, a different track/line for Violins 2, Viola, Cello, so on and so forth. This is the only way to really get your head around line writing and how each instrument interacts with one another. It is of the utmost importance that you envision composing and orchestration with each individual instrument, and not with just a large piano roll and lines. The dynamics will become static, and also, writing with an ensemble patch doubles up with many instruments. Middle C with strings will be played with both Violins 1 and 2, and also the Viola. It doesn't sound natural.
If you have an extremely tight deadline, I can understand using these. But I find this excuse is used more often than needed. Spend the extra time breaking the lines up into their corresponding instrument. I do understand using pre-made runs with some instruments, as the legato with runs is still iffy with most VST's.
In the end, it will payoff to write each note for each instrument. We are composers after all, so lets compose!