When you know you will not be using the computer for a longer period of time you can shut it off completely. As you wrote this is the best way to save energy.
But, when you want/have to do something on the computer every 30-45 minutes I would suggest to configure the power settings which can shutdown the hard disk(s), put the monitor in standby mode and put the computer into sleep so the energy consumption will be very little for the in-between-time. Monitor can be switched off manually as well of course, saving even more energy.
This article seems to be useful as well:
http://hotcoffeeblog.com/Cut_Your_Engergy_Bill_Dramatically_By_Doing_This.htm