beeswax, beeswax and more beeswax. Ascham talks about weather proofing your bow and it all comes down to use a ton of wax, over and over again. That and wool bow bags.
following quote from Toxophilus; Roger Ascham 1545 (I think)
Phi. Well Toxophile, I see well you be cunninger in this gear than I: but put case that I have three or four such good bows, piked and dressed, as you now speak of, yet I do remember that many learned men do say, that it is easier to get a good thing, than to save and keep a good thing, wherfore if you can teach me as concerning that point, you have satisfied me plentifully as concerning a bow.
Tox. Truly it was the next thing that I would have come unto, for so the matter lay.
When you have brought your bow to such a point, as I spake of, than you must have an herden(?) or wollen cloth waxed, wherewith every day you must rub and chafe your bow, till it shine and glitter withal. Which thing shall cause it both to be clean, well favored, goodly of colour, and shall also bring as it were a crust, over it, that is to say, shall make it every where on the outside, so slippery and hard, that neither any wet or weather can enter to hurt it, nor yet any fret or pinch, be able to bite upon it: but that you shall do it great wrong before you break it. This must be done oftentimes but (e)specially when you come from shooting.
Beware also when you shoot, of your shaft heads, dagger, knives, or agglettes, lest they race your bow, a thing as I said before, both unseemely to look on, and also dangerous for frets. Take heed also of misty and dankish days, which shall hurt a bow, more than any rain. For then you must either always(s) rub it, or else leave shooting.
Your bowcase (this I did not promise to speak of, because it is without the nature of shooting, or else I should trouble me with other things infinite(ly) more: yet seeing it is a safeguard for the bow, something I will say of it your bowcase I say, if you ride forth, must neither be too wide for your bows, for so shall one clap upon another, and hurt them, nor yet so straight that scarce they can be thrust in, for that would lay them on side and wind them. A bowcase of leather is not the best, for that is ofttimes moist which hurteth the bows very much. Therfore I have seen good shooters which would have for every bow, a fere(?) case made of wollen cloth, and than you may put 3 or 4 of them so cased, into a leather case if you will. This wollen case shall both keep them in sunder, and also will keep a bow in his full strength, that it never give for any weather. At home these wood cases be very good for bows to stand in. But take heed that your bow stand not too near a stone wall, for that will make him moist and weak, nor yet to near any fire for that will make him short and brittle. And thus much as concerning the saving and keeping of our bow; now you shall hear what things ye must avoid, for fear of breaking your bow.