The best way to strum fast is by using flamenco techniques. Those strumming techniques are called rasgueado. There are several different patterns, but I'll describe the most common one for now.
So first, join your middle and ring finger (right hand) togheter. Strum down with those two finger, but keep your thump up. Then, strum down with your thumb. Finally, strum up with your thumb. So basically, you have to strum down(fingers)-strum down (thumb)-strum up (thumb). It makes fast triplets.
Remember that you need to stay relax and all the motion is generated by your wirst. Like I said, there are a lot of other techniques where you use up to five fingers. Do some research on flameco techniques (or ask a flamenco guitar player).
Here is a little video that I've done using advanced strumming techniques :
http://www.youtube.com/watch?v=7hTNMHeNXGc . Look at 1:33 to see the fast triplet rasgueado technique.
Feel free to ask questions if you need more explanations.