I see that someone has already mentioned the Vick's. It worked for my 15 mo DGD.
Also, if allergies are involved, you might try putting a plastic cover on her mattress and pillow.
For the mattress I put a mattress pad over the plastic, then the sheets. That makes the plastic not noticeable. For the pillow I put on the zip-on plastic cover, them cover with a cotton zip on cover, then the regular pillowcase.
I also tumble blankets in the dryer each week just to get the dust out. I don't wash them first every week, just tumble. I do wash when needed though.
If these help, it is definitely worth it, you will notice a big difference. All of the plastic covers and cotton zip-on cover for the pillow are available at discount stores such as Wal-Mart.