Just parroting what was said before:
Create a part that a string section would actually play
Use a sample set that actually sounds real and allows you to edit all the variability that strings have.
However, if you are just using the stock string sound on a keyboard, the first suggestion is your only hope.
The suggestion of playing all the strings individually is good in theory, but might not get you the desired result. I did an album where an entire orchestra was done like this. 70+ tracks of individual instruments.
The problem I ran into was, the samples of a single violin were miced from a close difference, while you would normally mic a string section from relatively far away. So I ended up with something that sounded like I tracked all the instruments individually, instead of recording the orchestra as a whole.
I had to eq the snot out of everything and do a lot of things with reverbs to get it to sound kind of like everything was played together in the same room at the same time.