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...