Make certain your dog is legally allowed. Not all areas allow dogs on hiking trails. Most National Parks, for instance, do not allow dogs on backcountry trails. Be sure to check with your local ranger or land manager regarding the rules and regulations for dogs. Most areas that do allow dogs require them to be leashed at all times with a six foot maximum length. Sure, I bend these rules from time to time by letting my dog off leash in certain areas. That said, I do so at my own risk of fines. I am also very mindful and respectful of the terrain, wildlife, and other hikers in the area (who might have dogs, too) when considering how stringently to follow leash laws.
Be certain your dog is in proper physical shape. Your dog should be well-accustomed to exercise and capable to walk the distances you plan to be hiking. If Fido is a couch potato, don’t expect him to automatically transform into a svelte trail hound just because he’s out in the woods “in his natural element.” Beyond those basics, you should also consider your dog's agility level. Difficult terrain that involves rock scrambling, climbing backcountry ladders, or completing difficult river crossings should be considered. Be sure to ask local rangers or land managers about any potential trail conditions that could present problems for dogs.
