To discourage overbrowned cookie bottoms (which can happen when an oven heats unevenly), insulate the baking sheet by placing it inside a second baking sheet of the same size. The thin layer of air between the sheets will protect the top sheet form getting too hot.
To revive crisp cookies that have softened, bake them for 5 to 10 minutes in a 300 degree oven. Let them cool completely before storing.
To help soft cookies keep their texture, store them in an airtight container with a ceramic brown sugar softener, or with a piece of apple on a piece of aluminum foil, or soft bread (remove the apple after 24 hours).
from Tips Cook Love, Sur La Table, Rick Rodgers, Andrews McMeel Publishing