One thing that helps our so-called hypoallergenic pillows (the cheap kind, which are frankly mostly polyfil (though I did replace their crappy fabric cover with either pillow ticking or dense twill), is to (a) double case the pillow in two all-cotton pillowcases, and (b) wash everything FAR more often than most people. Frex, we got in the habit of having four pillows for each kid, so that two would be in the wash or drying while the other two were in use, then swap. To keep on top of four people rotating pillows every day, I was doing a load of pillows and pillowcases for SOMEBODY every day of the week.

Laundry soap is VITALLY important if someone has asthma or allergies-- certain chemicals can cause reactions in non-asthmatics, and even traces of the wrong soap in the pillowcase is mayhem on an asthmatic's lungs.

SILK pillowcases are a very, very expensive option, but they act more like a dust filter, keeping the bits of fiber from the cotton pillowcase and the pillow itself contained better than anything else I've found.