Echoing that the cold water trick will likely work, it was an old breeder's technique if the tie needed to be broken in an emergency (ie. virgin or inexperienced bitch was either in genuine pain or just overreacting and flailing about, but enough to be putting the breeding pair in danger of injury). For training, I can't say enough good things about Elypse art toys. They're the only thing I know of that mimics the real anatomy and physiology of a dog. I.e. the inflatable bulb goes in small then pumps to full diameter after its inside you. That is essential for training. Ammo-shell style anal dilators work well too. Get the "kit", the one that has several dilator toys in graduated sizes. When you can take the largest dilator and/or pump an Elypse 10-12x inside you on a regular basis, you should be ready for a real knot.
A footnote on the breeder's cold water trick for mitigating danger of injury: for our kind of danger, ie. imminent threat of being caught, like somebody's coming down the hall and about to swing that door wide open on you two, and you're far more worried about saving the life & happiness of you and your dog at the expense of just a mere few seconds of his discomfort. I say, in that circumstance, if you desperately need to break a tie in a pinch, and you have nothing else available at hand, you can use just that: a pinch in a pinch with your hands. A nose pinch to be specific. Grip his muzzle firmly over the top and down near the nostrils. He'll likely be panting you so you might need to lace your fingers under the jaw to close it. Do this quickly and with a very authoritarian, confident, no-nonsense attitude. Revert to alpha, lock eyes, and stare him down as you grab and hold. Do not break the stare. Take on a corrective, angry body language like you're issuing a correction for serious misbehavior. Don't worry, you're not suffocating him. This is not a choke hold. He can still breathe through his nose, but you are making that more difficult, and this difficulty equates to discomfort. The discomfort coupled with the sudden surprise/worry of his owner being angry at him should be enough to wilt down his erection. Basically, your aim is a distraction or misdirection technique. He was standing there on cloud 9, in the zone, on a wave of relaxed pleasure with you, but now he's suddenly anxious, worried that he's done wrong and made you angry. His surprise at the sudden reversal of attitude from you is what accomplishes it -- not any actual choking. Phrased another way, its more about communicating a serious correction in body language than causing any real physical harm. The 'shock' of his owner being angry is the boner killer here. It wrecks the mood rapidly, where a mere verbal scolding wouldn't suffice, or would take too long when seconds count. Its the sudden escalation of the correction taken all the way to 10 out of the gate, that is key. But be ready to follow him around on your knees as he may try to back away and you'll be tugged with him in those few moments before the knot subsides enough to decouple you two.
For background, this worked on a pit bull who is very friendly, and it had zero permanent consequence -- he was not at all reluctant to mount next time. He was more than happy to go right back to routine sex as if nothing happened. That said, I of course cannot guarantee it'll work on every dog. Disclaimer: obviously this is strictly 11th hour emergency only. Do not make habit of using, abusing, or overusing this technique. For two prime reasons: 1) it goes without saying this is unfairly cheating the dog out of the remainder of his orgasm and could make sex unpleasant, discouraging future acts. The onus is on you to toy-train yourself to handle that orgasm, ie. toying on a regular basis til you can confidently accept the full duration of a tie; and 2) if you repeat this pinch, you may condition him to expect it. The element of surprise is key to the functionality of the technique to begin with. Remove the surprise element, and it may become ineffective, such that his knot may fail to subside when the danger is real and the tie must be broken immediately. Or I could imagine with a more confident/aggressive male that it may trigger counter-aggression from him.
I share it here as a pearl of wisdom, an easter egg of sorts, to the community in case somebody ever finds themselves in an absolutely dire predicament. As in, your life and his are depending on it, and both of you are a few seconds away from disaster. It should be looked upon as a last-ditch before you forcefully rip yourself away and accept the injury. Think of it as the equivalent of breaking that fire extinguisher glass: it should only be done once, when there is no other way. For lesser emergencies, there's the diligence in preparation of bringing a bucket of ice water to your bedroom beforehand, because an ounce of prevention is worth a pound of cure.
Redrocket2085 said, inflatable.
let us know how you get on 
