Gonna copypaste some parts of it, here is the whole article: Quick Tip: Getting Realism in Orchestral Music
Make Velocity Changes

After you’ve loaded up the RR samples try to edit the velocity of each tone. Imagine for a while that you are a conductor, standing in front of around 60 players (strings only). When they start playing, they won’t play two identical notes – especially when it comes to volume.
So, going back to the MIDI orchestration, when you make slight velocity edits, you can add additional realism to your music. Sometimes this could take much of your time and you can use some helpful tools like randomizer plugins.
Humanize

The humanize technique can help you achieve the “live” sound of your music. This helps you escape the quantize-jail, and add slight rhythmic MIDI notes as it would be played by a live player.
No human player can possibly play as exact as a computer. Remember those 60 players we were talking about? It isn’t possible for them all to start playing at the exact time – every time one of them would be a bit late or early. And this is that magic that we all feel when we go to a concert (whether we’re listening to Haydn or U2).
