Dog Backpacks
Making the dog carry his own stuff remains one of the better ideas in hiking, and plenty of dogs enjoy having a job.
What has become clearer since this was written is the loading. The old rule of thumb was a quarter to a third of the dog's bodyweight, and current veterinary advice is considerably more conservative: start around ten per cent and treat twenty-five per cent as an upper limit for a fit, mature, well-conditioned dog on easy ground. Less for a young dog whose growth plates have not closed, less again for short-legged and flat-faced breeds.
Fit matters the same way it does for people. The weight should sit over the shoulders and ribs, not the lower back. Nothing should rub in the armpit, and the dog should be able to move at a full trot without the panniers swinging.
Build up to it. A few short walks with an empty pack, then a light load, over a couple of weeks. A dog that has never worn one and is loaded for a full day will not tell you it hurts until afterwards.
What to put in it: the dog's own water, a collapsible bowl, food, and waste bags. Not your camera. Keep anything hard or sharp out, and balance the two sides, because a lopsided pack is how a dog gets a sore shoulder.
And check where you are going allows dogs at all. Many national parks in the US restrict them on trails, which catches people out; national forest and BLM land is usually more relaxed.