# Create your superuser
$ mongo
> use admin
> db.createUser({user:"someadmin",pwd:"secret", roles:[{role:"root",db:"admin"}]})
> exit

# Alias for convenience (optional and at your own risk)
$ echo 'alias mongo="mongo --port 27017 -u someadmin -p secret --authenticationDatabase admin"' >> ~/.bash_profile
$ source ~/.bash_profile

# Add user to your DB
$ mongo
> use some_db
> db.createUser(
    {
      user: "mongouser",
      pwd: "someothersecret",
      roles: ["readWrite"]
    }
)

# If you get locked out, start over
sudo service mongod stop
sudo mv /data/admin.* .  # for backup
sudo service mongod start