Shutoff your HTPC when not in use!
These features have been widely available in Mythtv since at least 2008 but in the last couple weeks I have finally been able to implement ACPI shutdown/wakeup for my Mythtv server. Using the information found here I have setup my backend to power off when not recording a show. The system automatically sets the time of the next recording and will wake up 5 minutes before hand. Also, if there is a show starting within the next 90 minutes it will not shut itself off. Otherwise, the system will automatically turn itself off after 20 minutes of idleness. All of the specific time values are user-definable and can be set to whatever you want. I've had my setup running this way now for about 3 weeks and it has been flawless for me (other than the one time that the BIOS hiccupped when detecting hardware and prevented a proper boot sequence).
There are still a couple of areas that I need to improve. I would like to set the system to suspend instead of shutdown and I need to set the frontend systems to wake the backend when starting up. I'm sure that I'll be tweaking the various time settings for a little while too, but I am very happy with this setup right now.
If you haven't already done something similar in your HTPC software you should definitely consider looking into it. What started me down this path? I looked at last month's electric bill. I've been meaning to do it anyway but that was enough to push me along.
Good luck and leave a comment with your experiences if you decide to do this with your system.