When it appears that you have killed the monster, NEVER check to see if it's really dead. It isn't.
If you find that your house is built upon or near a cemetery, was once a church, had previous inhabitants who went mad or committed suicide or died in some horrible fashion, or had inhabitants who performed satanic practices in your house, move immediately.
Never read a book of demon summoning aloud, even as a joke.
Do not search the basement, especially if the power has just gone out.
If your children speak to you in Latin or any other language which they should not know, or if they speak to you using a voice other than their own, shoot them immediately. It will save you a lot of grief in the long run. NOTE: It will probably take several rounds to kill them, so be prepared.
When you have the benefit of a group of people, NEVER pair off and go it alone.
As a general rule, don't solve puzzles that open portals to Hell.
If you're searching for something that caused a noise and find out that it's just the cat, leave the room immediately if you value your life.
If you find a town that looks deserted, it's probably for a reason. Take the hint and leave now.
If you're running from the monster, expect to trip or fall down at least twice; more if you are of a female. Even though you may be faster than the monster, you can be sure that it WILL catch you.