In honor of the last four weeks of sickness around our house.
Mucous – (adj.) describing the cells and tissues that produce mucus.
Mucus – (n.) the slime produced by mucous cells and tissues. Often ending up in a tissue, come to think of it.
This one shouldn't be too hard to remember because -ous is such an obvious adjective suffix, as in "viscous," "infectious,"or, well, "obvious." So even if we can't remember the correct spelling, hopefully we can remember the part of speech we're talking about, can't we?