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:
sudo touch /.readwrite
The hidden /etc/rc boot script checks for the existence of the file, and if found, the drive is mounted read/write automatically. To create this file, you need to mount read-write first using one of the one-liners below.
Note that /Users, /etc, /tmp, and /var are mounted on the Scratch volume, which is always writable.
Alternative One Liners
sudo mount -uw /
sudo mount -ur /
I prefer to leave it mounted read only when I'm not writing