Chapstick consistency patch lube (beeswax and fat blend, fat could be olive or coconut oil or mink or neatsfoot oil) smeared around the edges of the closed pan. Then a greased leather lock cover (cow's knee) with a thick smear of the same lube around the barrel where the leading edge of the cover lies. A blob of same in the corner formed by the verticle side flat of the barrel and the horizontal top flat edge of the stock just forward of the lock . These last two are meant to help prevent water from running back down the outside of the barrel, especially down the "rain gutter" formed by that corner, and into the lock.
I see many people doing some or all of this, but then "frequently check/change the prime." I do not do this. If everything went in dry and was sealed while dry,,, why risk exposing it to wet weather?