Just line a jug with thin white cotton material and put a couple of cups of kefir in it. Tie the ends up with string and attach to a wooden spoon and suspend over a jar. Voila whey (and cream cheese)!
Dom's kefir site suggests hanging for 24 hours to get Kefir cream cheese but I have just found a website that suggests making Kefir sour cream by putting the cotton fabric in a sieve and letting it drip for 12 hours. Very good stuff and a bit more versatile than cheese.