Get a really good zombie costume. Scariest zombie costume one can find.
Turn off all the lights, so it's completely dark in the windows.
Hopefully, the house will be rather secluded. Off the road, sort of in a forest. About as creepy as it can get, yet make it appearent that it's decorated for halloween. I mean, who trick-or-treats at a house that obviously doesn't seem to celebrate halloween?
Get a buddy, and have him tear up his clothes, use cosmetics to make himself looked scratched up. So he looks like he got in a fight with some sort of bear or something.
Now, here come the kids. The kids that seem to be under the impression that this house is a good place to get candy.
They knock on the door.
Have your buddy knock on the other side. A desperate kind of "Get me out of here, I'm gonna die!!" knock. Perhaps some screaming for help. If they don't open the door after 10 seconds, have him scream in a way that says plainly "Oh my god, my organs are being torn out!!"
Have your buddy get on his knees and lean against the door, and wait for a bit. If the kids don't leave, and decide they wanna open the door, have your buddy fall to the ground, pretending to be dead as a doornail. Make your grand entrance as the big evil zombie. Make roar-moans and stuff. Try to fake twitching if you can. Like you're having a seizure. I find large amounts of twitching to be freaky.
If the kids don't run for the hills, present each of them with a snickers bar, congratulate them on their intelligence/bravory/leg-lockedness, and bid them a happy halloween as you wave merrily at them.