To have the boot drive automatically mounted as read/write, create a file at the root of the drive named .readwrite; you can create it with this command:
The hidden /etc/rc boot script checks for the existence of the file, and if found, the drive is mounted read/write automatically.
Alternative One Liners
mount -o remount,rw /dev/disk0s3 /
mount -o remount,ro /dev/disk0s3 /
I prefer to leave it mounted read only when im not writing