you know those pint glasses you usually get Guiness in? fill them with coffee, really strong coffee. if that doesn't work, soak your fingers and/or your tongue in espresso shots.
ok, seriously, as has already been mentioned, practice, practice, practice. I'll typically work on a tune at a slow to medium pace, working out phrases, ornaments and the overall feel of it. After working on it for awhile, I'll speed it up little by little. I rarely use a metronome for whistle playing (probably because I get enough of the tick-tock-tick-tock in cello practice!), but it's certainly useful, and seems to be good training for developing an inner sense of tempo (not to mention the old fashioned foot tapping/stamping method). Every now and then I'll play a tune alot faster than I practice it, not worrying about mistakes too much, just to get the feel of playing at a fast tempo, especially the feeling of moving the fingers much more quickly. Then, I'll go back to a slower tempo and work on the parts that need it. Something else to consider: playing the whistle (as with any instrument) is a physical activity, and I sometimes forget to pay as much attention as I should to the tension that sometimes builds up. I find this is especially true when playing low whistles--tension in the neck and mouth, as well as the fingers, so I try to be conscious of the degree of tension in my playing, as well as taking breaks if I'm playing for an extended period of time. Some of this carries over from cello playing, which of course is an altogether different instrument, but the principle of playing with the least amount of tension possible is certainly universal. Finally, breathing from the diaphragm is essential, even moreso when playing larger whistles. My $.02 anyway.
p.s. a mantra I used to hear from one of my former teachers: practice doesn't make perfect, but the right kind of practice will get you closer.
