Training a Jack Russell terrier requires special attention to digging, socialization, snapping, and barking.
Many Jack Russell terriers take to digging in the backyard, and can create large holes against fences or in flower beds. The most effective way of curbing the dog's destructive digging is curbing it into an acceptable area. Choose an area of the yard where the dog is allowed to dig, and make digging there a shared activity. Bury bones and toys and praise the dog when he digs them up, and scold the dog when he attempts to dig elsewhere. It's important not to let the dog into the back yard unsupervised for a week or two, until the training has taken hold. Once it has, most dogs will continue to stick to their spot for the rest of their life with only occasional reminders. If giving the dog a digging area isn't an option, try startling or scolding the dog when he starts to dig. Shock collars, bad tasting sprays, and an aluminum can full of marbles are all possible sources of unpleasant feedback that an owner can use when a dog starts to dig.
Some dogs are prone to snapping when excited. Make it clear to the dog that this is wrong by scolding him, and learn to recognize the difference between snapping from aggression and snapping due to excitement. A snappy dog might never fully curb its urge to snap when excited, and the owner will have to learn the point at which rough-housing needs to stop. Always supervise a child playing with a snappy dog, and don't let the dog get so excited it could possibly bite a child.
From the instinct to kill rodents can come an instinct to hate the family cat. Many a child's escaped hamster has fallen prey to the family dog, even in situations where the dog has never shown aggression before that and everyone is shocked. Jack Russell terrier's can be dissuaded from barking at squirrels through scolding, removing their line of site out the window, a shock collar or arming the master with a squirt bottle, but it's important to realize when purchasing a terrier that some dogs might never be able to live peacefully with a cat. Raising the Jack Russell around a cat drastically raises the chances that they can be a family together.
On that note, some dogs are also aggressive towards other dogs. Socialize a Jack Russell terrier early and often by enrolling him in doggie day care sessions, taking him to a dog park, or visiting a friend with dogs. Never let a dog go for more than six months without playing with another dog, and aim to have him meet at least one new dog every 2 weeks for the first year of his life. A Jack Russell who cannot tolerate another dog even with proper socialization is rare, but without the socialization aggression is common.
Hand in hand with the Jack Russell terrier's intelligence comes its attitude. This attitude is a delight most of the time, but gives puppy training a setback for some dogs at about 6 months. It's important that the dog understands who is in charge of the house and that there are some things the owner will not accept. Signs of a rebelling dog include peeing in the owners bed or shoes, looking his owner in the eye while being bad, or performing a misdeed (like trash diving) only on days when he is annoyed. Continue to reinforce the dog's training, and use a deep commanding voice when scolding him. Sometimes it can help to push a dog to its back on the floor, scold him, and glare at him until he acknowledges his position and his owner's anger by looking away. Reserve this sort of display for incidents where the dog was obviously striking back at someone, as opposed to just forgetting his training. It can be helpful to learn more about dominance and pack order if the dog continues to be difficult for more than a few months.
The most important weapon in the arsenal of an owner training a Jack Russell puppy is exercise and play. These dogs need hours of interaction and play every day. They need a hobby, attention, and challenges. Teach them to play Frisbee early, talk to them when doing chores, and stock up on puzzle toys early, or they will find what they need in incessant barking, bad deeds, and ingenious ways to escape over the fence.
Finally, never be afraid to call a professional. Jack Russell terriers are not good dogs for the first time owner (hopefully the owner realized this before buying one). Their strength of personality, intelligence, and need for interaction create surprises for every master, even experienced ones. A fresh perspective and a fresh face can do nearly as much as the expertise of a professional when it comes to training a problem dog or breaking a bad habit.