Keep candy out of reach, especially if it contains chocolate or xylitol (common in sugar-free candies and gum);
Make sure your pet has a microchip, collar and ID tag in case of escape;
Keep lit candles/jack-o-lanterns and glow sticks/jewelry out of reach
If putting your pet in costume, make sure it fits properly, is comfortable, doesn't have any pieces easily chewed off, and doesn't interfere with your pet's sight, hearing, breathing, opening its mouth, or moving. Give your pet time to get accustomed to the costume before Halloween, and never leave your him or her unsupervised while in costume;
If your pet is wary of strangers or might bite from stress or fear, put him/her in another room during trick-or-treating hours;
Keep your pet inside.
Excepted from a public service courtesy of the AVMA.