Read this:
http://www.wikihow.com/Clear-a-Building-With-a-Firearm
I'm often surprised at the quality of such wiki-articles...
Also, #1 buckshot will get to the fbi-required penetration depth of 12" in ballistic gelatin, with around 30% more disruption of tissue (more pellets) compared to #00... definitely use no less than #1, and keep in mind that birdshot is for little birdies, and will not stop an attacker.
Here's a cool site, with gelatin info for shotgun loads:
http://www.tacticalshotgun.ca/ballistics_shotgun.html
Lastly: practice, practice, practice! Clear your house with your shotgun during the day and night, just like you would if the threat were real... live the boy-scout motto and Be Prepared ^_^