Is there anything one should apply to mounts from time to time to keep the skins in good condition?

A good dusting every now and then is usually all that is needed. You can also gently wipe with a damp cloth in the direction of the hair. A gentle vaccuum can also be used. There is a poduct called "Taxidermy Magic" that can be sprayed on a cloth or on the mounts and wiped down. It will add a bit of sheen to the hair. Be careful if you use this product that you do not get any overspray on the floor as it makes the floor very, very slick. A drop of windex on a Q-tip can be used on the glass eyes.

These things are all cosmetic maintenance. If the skins were tanned properly they should stay in good condition as long as they are kept in a climate controlled setting, and protected from insects and rodents.