Using Vaseline will ensure that the protectors deteriorate much faster than normal - especially if you don't wash it off immediately and thoroughly.
Fairy liquid (or any other thick detergent - not the really cheap 'own brands') works beautifully, as described, as does KY jelly, or lambing/calving lubricant, which is virtually the same as KY and which you can buy in litre and larger containers at the agricultural supplies store.