hi sally well first of all, and Im sure you already know, if your dog is stung in the mouth, straight off to the vets asap.
For relief of pain, first soak the sting site in vinegar, then hold ice cube on the spot for as long as possible. Hope this advice will not be needed though!
Give a Piriton as soon as you know that a sting has happened. and then if it is in the mouth or throat go to the vet, tell him you have given antihistimin. A dog of 20kg can be given up to 4 tablets. I always carry some tabs with me they work like magic. If it is a bee sting try to remove the sting as it goes on pumping poison even if the bee has died.