Do this
Rinse the caps quickly. Lay them gills down in cold water. Press them under with a plate, then chill 8 to 24 hours.
Why it works
Dried caps float and dry out on top. Gills down, the thin side drinks first. The plate keeps every part in the water.
When to skip it
Skip the long soak for thin, sliced shiitake. They soften in about 20 minutes of cold water.