I like to use cervix with both endo- and ecto-cervix as the control.  The 
stratified squamous epithelium of the ectocervix contains a lot of glycogen 
which will disappear with diastase, while the mucinous glands of the 
endocervix will still stain PAS positive after the digestion.  You have a 
positive and negative control in the same section so to speak.

Freida Carson