This may sound a little weird but after seeing the comments on putting dryer sheets in between clothes to stop mold, is there any special way that you treat your mattresses? What if you have a memory foam? They kinda absorb.
We use mattress protectors with zippers that don't allow anything in or out on the memory foam and the traditional spring mattress. I suppose we could also put some silica gel packets inside to absorb any latent humidity.
I have a memory foam mattress, havent had any problems with mildew on it, but it seems to ruin lots of other things. Even rubber bands.
I have the foam toppers on my mattresses and they have held up really well. They yellow with age, but as far as mold or stains they are fine. I do have thick mattress pads over them at all times. If I had a whole memory foam mattress I would do like Sucbadoo and use a zipper cover. On a side note, the best foam topper I have had is the ISO Cool. Have tried three others and that one is by far the best.