Here are the commands needed to set a service to start on boot.
Tested on Ubuntu Server 14.04 but should work just fine on older versions as well.
root@t605:/# update-rc.d ntp enable
update-rc.d: warning: start runlevel arguments (none) do not match ntp Default-Start values (2 3 4 5)
update-rc.d: warning: stop runlevel arguments (none) do not match ntp Default-Stop values (1)
Enabling system startup links for /etc/init.d/ntp ...
Removing any system startup links for /etc/init.d/ntp ...
/etc/rc1.d/K77ntp
/etc/rc2.d/S23ntp
/etc/rc3.d/S23ntp
/etc/rc4.d/S23ntp
/etc/rc5.d/S23ntp
Adding system startup for /etc/init.d/ntp ...
/etc/rc1.d/K77ntp -> ../init.d/ntp
/etc/rc2.d/S23ntp -> ../init.d/ntp
/etc/rc3.d/S23ntp -> ../init.d/ntp
/etc/rc4.d/S23ntp -> ../init.d/ntp
/etc/rc5.d/S23ntp -> ../init.d/ntp
usage: update-rc.d [-n] [-f] <basename> remove
update-rc.d [-n] <basename> defaults [NN | SS KK]
update-rc.d [-n] <basename> start|stop NN runlvl [runlvl] [...] .
update-rc.d [-n] <basename> disable|enable [S|2|3|4|5]
-n: not really
-f: force
The disable|enable API is not stable and might change in the future.