Are you talking skates or Rollerblades (inline skates)? I've been rollerblading for probably ten years and I'm still not the best at stopping. But, for blades, you're supposed to push your heel down and lift the front of your foot up. It can help to squat down a bit, bending at the knee.
As with regular roller skates, you can also point the front of your skates toward each other so your feet form a V.
I go blading mainly along the beach, so when all else fails I can hop off the path and into the sand, but this is really the worst case scenario. Sand in a skate's bearings are not a good thing.