Miso comes in many varieties; for the following soup, I favor a dark miso. See also my more recent Miso++ recipe, which uses lighter mushrooms than the shiitake below.
Chicken broth worked well when I had a head cold, though is too salty otherwise. Japanese soup stock (usually based on Bonito) should work well.
Fresh ginger will keep longer if frozen; simply shave off what is needed with a grater.
Adding in seaweed also works, though the traditional miso soup should not be too crowded.
If sick, grinding the shiitake up in a food processor first makes for easier eating.