Doubly tragic is the fact that I've lent all my animal magic books to my friend who's a struggling veterinary surgeon. So all we are left with the scraps of the Internet and I'm afraid I do not have the time or the resources to test whether these work.
Over on Spells of Magic (www.spellsofmagic.com/spells/spiritual_spells/conjuring_spel) they have a simple "Dog Transformation Spell" to transform yourself or someone else into a dog. You need a photo of the desired dog you wish to turn summon into being (with the name of the dog written on the back of it) and plenty of white candles with which you can make a circle.
If you're transforming someone: Place the person in the circle of candles. Then take the photo of the dog and burn it while reciting "This person standing here, I transform thee, into the dog I am burning, almost immediately. Through time and space, I transform thee, into the animal I change them to be".
If you're transforming yourself: Same as above but putting yourself in the circle and reciting "Hear me now, answer my plea, I transform myself into the dog I wish to be. On this night/day (delete where applicable) and in this hour, I become a dog with all my power.
If this doesn't work (and it probably won't) and you still have a really strong urge to transform yourself into a member of the canine family, I'd advise a brief chat with your local doctor.
Of course if you talk to a bitter divorced woman, she'll say that a few beers turn men into dogs while a lot of beers will turn them into pigs.