How about a couple of video-less hints?
Typically, if the shuffle is carrying itself (that is to say, you don't need to drive it with your handsticks at all), you've got enough speed for a sprinkler. Play with getting your diabolo's spinning as quickly as you can make them and then trying the sprinkler - a really fast shuffle should have the diabolos nearly rocketing out of your string and banging into your handstick. Experiment with different speeds as well and see what suits your fancy.
And from our illustrious admin:
You need A LOT of speed. Generally you will need to whip the diabolos to get enough speed for a good sprinkler. Unless you're Finnish apparently - check out some of the triple sprinklers these guys pull off with no whipping!
Other than that, the motion you use to get into a sprinkler can make a difference. Ultimately, it comes down to practice. Mostly practice at getting 2 diabolos to a very high speed (which is a good skill to have for all tricks).
Sean
-Duncan