Ooh =) This is my section baby! I probably like and listen to more ska that almost all of you guys. Here's some of the many ska bands that I have grown fond of:
Mustard Plug (my favorite)
Voodoo Glow Skulls
Reel Big Fish
Mighty Mighty Bosstones
Less Than Jake
Catch 22
Streetlight Manifesto
The Suicide Machines
Buck-O-Nine
The Aquabats
Blue Meanies
MU330
Assorted Jelly Beans
and many others...
The key to writing ska progressions is using, like they said earlier "3 note chords based on the bottom strings."
One thing I've noticed in writing ska riffs is that the chords usually only have subtle changes in them, meaning one or more notes stay the same when you change chords.
Timing is key when it comes to strumming. You have to be very quick with your chord changes and work on your finger positioning so its easier to make the changes. Ska strumming is usually off-beat strumming which is why up-stroking is he best technique.
It takes practice, like everything, but once you become familiar with the common ska chords, and the stroking, It shold come pretty easy for you.
Take it from the man who loves ska more than anything haha