Use more brown sugar than white sugar
Add an extra egg yolk
Use softened butter, not melted butter
Do not overmix the dough
Chill the dough before baking
Bake at a slightly lower temperature
Remove cookies from the oven when the centers still look slightly underbaked
Let cookies cool on the baking sheet for a few minutes
Store cookies in an airtight container
Add a slice of bread to the container to help retain moisture