In a nutshell (I'm sure someone will supply a link to a very indepth description):
Mix 1 tablespoon of salt with just enough DW to make a paste.
Put this in a small container like a bottlecap
Put the bottlecap and your hygrometer in a ziplock style baggie with enogh free stadning air that neither touch the other
Now doublebag the whole thing
wait EIGHT hours
doun't cheat at 4, really wait 8
The exact humidity in the bag is now 75%
adjust your hygrometer accordingly
double check if necessary (yeah, wait 8 hours again)
you'll now have a calibrated hygrometer
There's a faq somewhere that explains it in detail, but I don't have the link handy. Basically the idea is that a saturated salt solution has a vapor pressure such that it will create a constant 75% humidity in a sealed container. So you can use this to calibrate your hygrometer. You put the hygrometer in a baggie with a soda cap (or other small cup-like substance) full of salt and a little water. You want just enough water to wet the salt, but not dissolve all of it. Seal the baggie and wait a few hours. When you check it again, your hygrometer should read 75%. If it doesn't, use a small screwdriver to turn the little calibration screw until it does. At that point you should be all betterbetter calibration-wise.