Natural Phrasing of Brass Sustains
Orchestral Brass players will always execute a natural decrescendo on sustained notes unless otherwise indicated by special dynamics. Many sample libraries come with absolutely flat sustains on the brass (in order to be loopable) and many people use them without giving them a natural dynamic shaping.
In order to raise the realism in your midi programming, make sure that you shape the sustains in a more natural way by decrescendoing them. This is especially important on endings of phrases but applies for more or less every longer note.