The Number One Tip to Help Create Realistic Orchestral Mock-ups
The main tip to get better at creating realistic mock-ups is to listen! Listen to as much orchestral music as possible. Especially from the masters. Listen to impressionist orchestral music like Ravel and Debussy. Arnold Bax and Delius. Closely hear how all the colors of the orchestra are playing and bouncing off each other. The levels of each section and individual instruments.
When you get a good ear with orchestral music, writing mock-ups become infinitely easier. You can quickly tell when a line is slightly off and how to fix it.
I hear many midi created music, with such wonderful compositions, but the virtual instruments just seem....off. An orchestra is an organism with different parts and sections. They all work together seamlessly. Listen closely to it and try to recreate it as best you can.