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Use an alternate entrance and exit. Don't give your cat any attention at the door. Try a pet-proofing barrier or spray. Lock your cat door. Teach your cat to sit. Annoy your cat. Spay or neuter your pet.
If your cat is constantly hanging out by the front door, waiting to make its escape when some unwitting person opens it, try exiting out a different door. For instance, instead of going in and out through the front door, try using the back or garage door. Another good option is to exit and enter via a door with an antechamber. After passing through the first door, close it firmly behind you and look about to ensure your feline friend hasn't followed you. If your cat does get past the first door, you'll notice and be able to put him or her back inside before exiting the secondary door which actually leads outside. When you have guests over, put your cat in another room until the festivities are over. This way, when people come through the main door, your cat won't be around to dart through it. If your cat thinks that being near the door is a place where you will pet or play with it, your cat will gravitate toward the door. If your cat is in the habit of greeting you and getting a friendly hello right when you walk in, break yourself of that habit. Don't even look at your cat until after you've taken your shoes off, removed your jacket, and moved away from the door. Instead, greet your cat warmly and pat it's head only in the living room, bedroom, or hallway. This way, the cat will learn to meet you at the place where it knows you'll give it attention. Do the same thing when you leave. Instead of bidding your cat farewell at the door, do so at your designated hello/goodbye spot. A pet barrier is a small device which emits a loud beeping sound when your cat approaches the door. The sound is triggered by a wireless device you attach to the cat's collar. When the cat approaches to close to the door, the sound will trigger, scaring the cat away. If your cat is insistent on approaching the door, he or she will receive a small, harmless static shock from the collar in order to keep them inside. In time, your cat will avoid going near the door altogether. A pet-proof spray is similar in purpose, if not design. Place the spray can near the door you want the cat to stay away from. Then, switch it on. The device will emit an irritating but harmless spray whenever your pet approaches the door. Be sure to turn the device off when you're not expecting to open the door through which the cat might disappear. If your cat is going in and out as it pleases because you have a cat door, utilize a lock or a sliding latch to prevent it from darting outside. If you do not have a cat door with a lock or sliding latch, you can easily attach one and open it during certain times of the day when you want to allow your cat to go outside. Choose a comfortable place for your cat to sit. A special kitty bed or carpeted perch are good options. Before opening the door to leave, bring your cat over to the designated sitting spot. Use a treat or a toy like a small bell to get the cat's attention. Once your cat is situated where you want it, say "Sit." Be firm, but not aggressive. Say it again several seconds later. Repeat this about ten times, and award a treat after three or four times. Don't worry about actually getting your cat to sit in a seated position as you would with a dog. As long as your cat removes itself to the place where it will remain when you exit your home upon hearing the command, you've accomplished what you needed to. Keep a spray bottle filled with water just outside your door. When you open the door to come in, open it just a crack so that you can see your cat waiting to run outside. Put the nozzle of the bottle through the crack and hit your kitty with a well-aimed squirt. It might take a few direct hits before your cat backs up. After a week or so of doing this, your cat will associate the door with getting sprayed by the squirt bottle and avoid going near the door. This method, unfortunately, only works when you're entering the home, not exiting. If you do it while exiting, your cat will associate you, not the door, with the irritation of getting sprayed by the water bottle, and it could create bad feelings between you. Alternately, as you enter your home, make loud noises like hissing, stomping, or knocking to get the cat away before opening the door. If your cat is not spayed or neutered, it will have the urge to wander about looking for potential mates more than a cat who is. If your cat is fixed, on the other hand, it will have no desire to breed and thus be more inclined to stay home. It's typically safe for cats as young as eight weeks old to be spayed or neutered, but you should always ask your vet if and when it is possible for your cat to be sterilized.