The foam degrades over time with normal use. As far as I understand, nothing can be done to prevent it.
Continue to buy new replacements as they wear out.
You can get them here (a little pricey IMHO):KOSS replacement pads.
Also, if memory serves, Radioshack may carry pad replacements that will fit the KSC 75s as well:Here ya go.
One final option - If your headphones were to accidentally break, you could send them back to KOSS along with $6 and they will mail you a brand new pair.