Retraining your dog is easiest when he is young. Always handle your puppy so that he becomes accustomed to being touched. When dogs are young, they usually like to be handled. Older dogs that were deprived of physical contact growing up sometimes have difficulty accepting physical affection. Start off gradually. For example, begin by just trying to touch your dog’s paws with the clipper, and build up the process each day, while giving constant praise an a small treat on occasion. A dog that has already had negative handling/grooming experiences will be harder to retrain. You will need to reverse the damage that has been caused by those bad experiences, and you can do this by starting off very slowly and making sure that your dog stays relaxed. If he begins to exhibit signs of stress, stop right away and allow him to calm down. Try your best to make it seem like a game: give your dog lots of attention and praise, pet him, and give him treats. Remember not to rush and not to insist if your dog shows anxious behavior.
Dogs display aggression for a purpose: they’re warning you to leave them alone, or pay the consequences. Although this is one way explaining how to rehabilitate aggressive behavior in a dog, if your dog just refuses to be groomed, despite all of your attempts, it would be a wise decision to let the professionals take over. The vet will clip your dog’s nails (be sure to tell the vet that your dog becomes aggressive when he sees clippers, so that he is prepared). You can also get your dog washed, brushed, etc by experienced professionals for a nominal fee (again, make them aware of your dog’s prior bad experiences with grooming beforehand!).