Having a strong dog means having a dog that you can take into most situations with confidence that their behaviour is going to be friendly controllable. It also means having a dog who can take the hard knocks in life and bounce back. This is often referred to as resilience, it is an important attribute to nurture in your dog because it will allow them to get the most out of life.
A strong dog is only as good as his foundation, a lot of your time should be devoted to choosing and socialising your dog. Too many people rush this aspect of dog husbandry and as soon as pressure is placed on an already wobbly foundation your dog falls apart, loses confidence and this is where problems arise. The key to a strong dog is a strong foundation of good breeding, consistent socialisation and motivational training.
The best place to start building a strong dog begins even before you bring him home. Ensure you are making informed choices about the breed of dog you would like, as well as being involved with the breeder in choosing a particular puppy. Meeting and getting to know your dogs mother (if possible father too) will give you a window into the genetics at play in your new dog.
If you are adopting an older dog it will mean spending as much time getting to know him before you bring him home and being prepared to address already formed habits. When you choose a dog try as best you can to be influenced by your head, not your heart, most problems between people and their dogs occur because of a miss-matched pairing.
Socialisation and confidence-building comes right after bringing your new dog home. Spend time with your dog building a bond and getting to know how he responds in certain situations. Become familiar with his likes and dislikes. Take your new dog to different places to allow him to observe the world and create positive associations with it. Always have some treats with you so you can reward confidence, and continue to socialise after puppy school and all the way through your dogs first three years of life and beyond.
Training certain skills will also create a strong dog who is reliable and predictable. The more cues a dog understands the more likely he will be included in your activities and the more he will be exposed to new and enjoyable experiences. Motivational training builds confidence in a dog because he can experiment with new behaviours and achieve a great high when he gets things right. A dog who is confident to try new behaviours is a dog who is not inhibited by his own anxiety. Try teaching your dog some new tricks or shaping certain behaviours, these are great strengthening exercises for you and your dog.
If you already have a dog who lacks confidence or is anxious you can still practice all of the above tips, just work slower. Work on confidence-building in strange settings by pairing something anxiety-provoking with something wonderful like a handful of meaty treats. Work on training certain behaviours, even tricks, so you can use them to reduce stress. As long as you are taking it slowly your dog should respond to these things well.
Realise that if you already have a dog who has problems they may never be what you wanted them to be, however, take satisfaction by remembering how far they have come. A strong dog should only be measured by their upbringing and temperament, and this will be your benchmark. Do not make the mistake of measuring your dog against another, they are all different.
Training a dog is a marathon, not a sprint, take your time and be thorough from the start, never miss an opportunity to reward confidence and always remember how far you have both come.