It's a terrifying feeling to have a dog on a leash one moment, and the next moment to have a dog loose without tags while an empty leash and collar dangle in your hands. Some very clever, fearful, or aggressive dogs make a habit out of pulling their collars off so they can escape. Most dogs who are able to pull their collars off are breeds with narrow heads but if the a collar is loose enough any breed can learn this "trick" to freedom. If you have a dog who frequently tries to escape her collar you have a few options:
The easiest solution to keep the "slippery" dog leashed securely is a martingale collar. A martingale collar tightens in circumference, settling just behind the ears when a dog pulls backwards- making it impossible for the dog to pull out of a collar. When we are working with a rescue dog that has a tendency to slip our of their collar, we keep them in a martingale style collar all the time unless they are playing with other dogs indoors or are crated (two situations when a martingale collar shouldn't be left on). Some people don't recommend keeping a martingale collar on 24/7 because a dog could theoretically get a leg caught, but if properly adjusted (so that when the leash is taut, the collar is snug but not choking) this is not a problem.
Tips for walking on a leash with a dog who slips his collar:
Reactive dogs (aggressive, scared, or just clever) may do better on a leash if you try thinking a step ahead. See a dog coming that will make your clever dog want to slip his collar and go play? A loud truck that may make your timid dog want to bolt? React before the dog does by redirecting the dog's attention. "Sit' is a magic word that can help in many situations. If the dog's brain is actively involved in listening to your words and putting his body in action to obey the command, it may distract him from the event or lessen the impact. Fearful dogs especially do well being redirected into a positive activity.
Dogs who slip their collar usually do so by backing out of their collar. If you sense your dog is about to try this do anything you can to position yourself behind the dog. If the dog tries to run backwards only to discover a leg or your foot behind their hindquarter they'll stop backing up, and thus stay secured.
If you can see the slip about to happen, do ANYTHING to urge your dog forward. Trust is a valued thing, particular between a rescue dog and owner, but if scaring your dog forward keeps them safe, it's worth it. If your positioned correctly, you can instead run forward with the dog. Some dogs might respond well encouraged to break into a run by your side in this situation. The collar slipper's instinct is to escape, and giving them the opportunity to run might fool them into feeling that urge satisfied long enough for you to regain control. If that doesn't work, run backwards with them. In order for a dog to slip out of a collar, opposing pressure has to be exerted via the leash. If you can eliminate the tug of the leash then you should be able to prevent the escape of your dog. This may mean turning and running backwards as soon as the dog begins to back up (careful! Most timid dogs will interpret this as chasing and try to flee. Attempt to get the dog turned, so they are running forward or just to physically catch the dog with your hands)
When all else fails, dropping the leash and letting the dog run with leash and collar in tow might be the best, last option for when you have a dog trying to slip its non martingale collar. (Again, a martingale collar prevents all these issues) With leash dropped, your loose dog is at least wearing tags and something to be caught by - which is a bad situation but not as severe as your dog being loose without identification.
If you choose not to use a martingale collar, you should consider putting your dog in two collars whenever you leave the house- one collar with the leash attached and a second collar below, with ID and not attached to the leash, that would stay on if the collar attached to the leash was escaped. If you have a dog that absolutely escapes at every opportunity you may want to consider putting a dog T-shirt on the dog whenever he leaves the house with this written boldly on the side: "If I am alone, I've escaped! Call 555-1212"