Think about your slumping temperature
I have not had to re-apply kiln wahs to my moulds for years. If your kiln wash is chipping off after a few firings, you should consider at least two things:
Are you firing higher than you need? usually 630 – 677C is enough with a longer or shorter soak is enough.
Are you putting a lot of kiln wash on the mould? A thin layer is most often better than a number and thicker layers.
My method of preparation is here: http://glasstips.blogspot.co.uk/2012/10/mould-preparation.html