Hello! Here are some important things:
1. Use separate tracks for different string groups, just like in a real orchestra -- violins 1-2, violas, cellos, basses
2. Use volume envelopes on each track
3. Switch between different articulations (legato, staccato, pizzicato etc.)
4. Use randomize on starts of notes, so that different tracks don't play notes exactly on the same time (which sounds unnatural)
5. Use orchestration rules. If you're not really familiar with this, get some orchestra scores and study them.
As for sample libraries -- of course, you may be aiming to get some huge orchestra library, but in many cases orchestra presets included with Logic will do the job, and you won't be spending money on additional libraries and hassle with many GBytes of samples. I don't have Logic X, only Logic 9, but I suppose orchestra presets are the same in X.
Here's an example of what I did with Logic 9 factory string presets:
http://yadi.sk/d/PM9fyFT_9xE8K
Didn't have time to randomize stuff, but volume envelopes are there.