Hint 1: Play higher on the keyboard than the guitar and other keys. Strings too low get muddy fast.
Hint 2: Get out of the closed-voiced triad habit. Strings are usually one or two-note pads or counter-melodies, sometimes spaced out over an octave or more. Mostly one-hand voicings (usually right, unless you are doing a low melody line as a special 'cello' effect).
Of course if you are the main keyboard player, using a layered sound like piano+slow strings these don't apply as much.
As another poster said, listen to recordings of string lines for ideas...