1. Don’t feed your pets Halloween candy, especially if it contains chocolate or xylitol (a common sugar substitute found in sugar-free candies and gum);
2. Make sure your pet is properly identified (microchip, collar and ID tag) in case s/he escapes through the open door while you’re distracted with trick-or-treaters;
3. Keep lit candles and jack-o-lanterns out of reach of pets;
4. If you plan to put a costume on your pet, make sure it fits properly and is comfortable, doesn’t have any pieces that can easily be chewed off, and doesn’t interfere with your pet’s sight, hearing, breathing, opening its mouth, or moving. Take time to get your pet accustomed to the costume before Halloween, and never leave your pet unsupervised while he/she is wearing a costume;
5. Keep glow sticks and glow jewelry away from your pets. Although the liquid in these products isn’t likely toxic, it tastes really bad and makes pets salivate excessively and act strangely;
6. If your pet is wary of strangers or has a tendency to bite, put him/her in another room during trick-or-treating hours or provide him/her with a safe hiding place;
7. Keep your pet inside.

Source AVMA