Decongestants, especially antihistamines might actually make this worse as they reduce
secretions, including saliva. Not a whole lot but that is their modus operandi. Solution is
more water or whatever liquid you prefer. If you are hydrated the salivary glands have
lots to work with and the saliva stays more liquid. Hill climbing increases your respiratory
rate and dries the mouth out faster, concentrating the mucoid content into the goo you
have problems with. Drink more often.
FWIW my cat developed 'thick mucus laden saliva' recently when we tried to stuff a flagyl
tab down its throat. Kind of funny but utlimately the cat defeated us as neither wanted to
poke a finger far enough down that it couldn't get the pill back out and the pill holder
wasn't much help. Cat is prone to bite.