OpenSSH can be replaced by dropbear to save memory. Stop OpenSSH and prevent it from running at start:

touch /etc/ssh/sshd_not_to_be_run service ssh stop

Install Dropbear

apt-get install dropbear

Set NO_START=0 in /etc/default/dropbear so dropbear can run as a daemon:

vi /etc/default/dropbear

Start Dropbear:

/etc/init.d/dropbear start