Never much played Ska so best I can say is emphasize on the upstroke. For instance, if you were playing over a 4/4. Standard riffing might have to empahsize on the downstroke at the 1 and the 3 ( 1 - 2 - 3 - 4 ) but with Ska, you are upstroking on the 2 and 4 ( 1 - 2 - 3 - 4 ).

Then again, I never really played it so I could be totally wrong.

So, here's a little vid that should give help. I seem to remember that Ska jumps from major chords to minor chords (and maybe some sevenths).

http://www.youtube.com/watch?v=ykb7EZfYz7g