The obvious starting point is panning. I get most orch samples are recorded in situ with the image as it would be in a full session recording, but I often find that samples sound perceptually narrower than the real thing would do in the same space, so don't be afraid to exaggerate the natural panning slightly. You can try it just with the default pan pots, or there are various utility plugins that will pan for you in different ways with eq, phase relationships etc - sometimes that can sound more natural, used sparingly.
Don't be afraid to build your sound world in a way that wouldn't be possible with a real trad orchestra layout - samples afford you the possibility of splitting more lines out of separate processing, so if there's a Violin 1 part that would sound better coming from the right when the rest of the section is left, go ahead and flip the panning.
The other part of the equation is layering. Fill in the sound gaps of your primary strings with different libs, different articulations... Anything to add more complexity and depth (obviously this is an art in itself and not as simple as just slapping more shit on top). Get it right and it adds to the feeling of a rich stereo image much more effectively than sticking Ozone on the master and cranking up the width.