A nervous dog can be dangerous. They have a tendency to snap out of fear, so it is important to calm your dog down. Firstly it is important to establish why your dog is nervous and anxious. It may be that your dog is simply highly strung, or that they were badly treated at one time resulting in a fear of humans or certain situations. Fireworks and other loud noises such as thunder can hurt the dog's sensitive ears causing them to become anxious and afraid. You need to eliminate the source of fear.
Remain calm yourself. Dog's pick up on fear and anxiousness, so if you, yourself, are nervous or afraid it will cause your dog's emotions to become heightened. Use a soothing tone when you talk to your dog. Don't jump around and shout, and ask any small children to leave the room as sudden movements and noise can upset the dog further.
If the fear is the result of previous bad treatment it is important to show him that not all people are like that. Socialize your dog with other humans and animals. Your vet surgery may be able to point you in the direction of a training group. This is not only to teach you how to handle a dog and to train your dog to be obedient but also helps them interact with other dogs of different sizes and ages.
Exercise is important too. Taking your dog to the local park and allowing them to run around not only lets them socialize, but also helps them release their energy. It is medically proven in humans that exercise can help combat depression, so it is bound to help nervous animals too.
Meditation may sound absurd, but playing gentle music and 'massaging' your dog can help them relax tremendously. There are classes in dog massage, as it is much more than simply stroking your pet and if done incorrectly can cause damage, but if you do not want to do this then just stroke them gently. Using a soft brush as well as your hand will help to groom them as well as relax them. It also helps to tone muscle.
There are pheromone diffusers available such as D.A.P (Dog Appeasing Pheromone) that are used the same way as a plug-in air freshener. Humans cannot smell them, but they emit a scent into the air that can help to relax dogs. It is recommended that you use these for a few weeks before the event that is likely to upset your dog, such as when fireworks are common. Some herbal remedies that you can buy from pet shops work in the same way. However, before giving your dog anything herbal check with your vet that it will not contraindicate any other medicines or conditions that your dog may have.