Add the following line to /etc/rc.local:

if [ -x /usr/local/bin/monit ]; then
       /usr/local/bin/monit
fi

On AIX add the following to /etc/rc.startup:

if [ -x /opt/freeware/bin/monit ]; then
       /opt/freeware/bin/monit -c /opt/freeware/etc/monitrc
fi

And the following to /etc/rc.shutdown:

if [ -x /opt/freeware/bin/monit ]; then
       /opt/freeware/bin/monit -c /opt/freeware/etc/monitrc quit
fi

Note: Sometimes, it is more useful to install monit to "/usr/local" on AIX also.