You need to drill a hole in your return tube than sits in the tank. This hole needs to be just under the water line when it normal operation. This way, when the power goes out, the water will siphon back via the return line, but it will "break" when the water gets down to the hole you drilled because it is drawing in air as well. When I turn my sump off, it back siphons for less than one second before air is exposed to the siphon break hole.
This is hard to explain but I hope you understand wha I am talking about. A picture is worth a thousand words so hopefully someone will have one. If not, when I come back from lunch I'll take a picture of my return line with the hole drilled in it and show you what I am talking about.
Also, be sure you don't have you sump operating with water all the way to the top of the container. Mine has water up to about 75% of its total capacity. I leave the other 25% as margin of error incase it was to overflow if the siphon break didn't work (unlikely however). This way, my sump could still handle the extra water.
