Achieving realism in strings when using sample libraries is critical for a nice musical result. Today there are great libraries (for samplers like Kontakt, VSL libraries etc) which are sounding fantastic. Most of them come with round robin sampling and settings like dynamics, expression, humanize etc. which add exceptional realism.
It is very important to take advantage of all of these. There are a few simple tips that help to keep realism to levels that no one could tell if it is a real human plays or it is a sample library.
The first is to perform the part rather than draw the notes in a midi clip. When familiarize yourself with a specific sample library and understand its response to various playing ways, it would be more easier to achieve the wanted feeling in the musical result. Also when performing I most of the times prefer to add some reverb to the string track to give me the impression of room or hall or whatever I need in each project. I turn the Round Robin option on, so each time I trigger the same note it plays another sampled version of this note.
After that I like to tweak the dynamics and expression of the strings continuous but subtly. For this I use the drawing tool of my DAW to automate those parameters. I prefer to draw arbitrary lines rather than quantized and repeating ones to get a more realistic result. In some libraries there may be more settings to help you to that way, like small variations in intonation and rhythm (humanize).
Finally it is good to review the track in conjunction with the rest score to ensure that those changes are fitting well.