Oh, I'm so sorry to hear about your precious pet. We had to have out cat put to sleep exactly one year ago today, so I totally understand how you feel!
Our cat had cancer of the jaw. We took him to speciality clinics, had him on chemotherapy and radiation, and it worked for awhile, but then the cancer just took over. Believe me, I know what a difficult decision it is to make! You want to keep your pet as long as you can, but don't want to let him suffer. Just how to decide when to let go? Its a decision only you can make, and obviously you've come to that conclusion. I agonized over the decision--did I even have the right to take his life, regardless of the circumstances? Well, its like I explained to my children, we're given domain over animals from God, its up to us to make responsible decisions for them. Something I read concerning euthanasia that helped me "...better a day too soon than a day too late". At first I didn't agree with that--I didn't want to take away any good time he had left, however, I didn't want his last memories to be pain and suffering. Its the responsible, loving thing to do. Your last gift to your pet is the most difficult, but the most caring.
Here's some websites that might help
www.rainbowbridge.com You can create a memorial for your pet here. Its $25/year. At first I didn't think it would help that much, but it does. I just visited his site tonight, its comforting to have someplace to go, even if its in cyberspace.
Also, there are several yahoo groups that I found helpful. They are for cats, but I'm sure there's similar ones for dogs. Go to Yahoo and type in canine cancer, canine grief support, I'm sure you'll find ones to suit your situation.
I do understand how gut wrenching this is for you, having gone through it myself. One thing someone told me was to be strong, don't cry in front of your pet, don't let him know you're sad or he won't be able to go in peace. Don't let his last memories be of your sadness. I don't know if you plan to be with your pet during the process or not, its your choice, but we stayed with our kitty. Actually, we had a vet come to the home to perform the euthanasia, but its hard to find someone to do that! He was sleeping in the bedroom, she came in, my husband picked him up and cuddled him while the vet gave him a quick injection in his hindquarters that he didn't even feel which rendered him quickly unconscious, then the vet found a vein in his leg, inserted an IV, and ran an infusion with the lethal drugs. It was all very quick and peaceful, I was holding his paw and it just went limp and I knew he was gone. Regardless of whether you decide to stay during the process or not, you need to know what happens so you can be at peace with it.
I'm a night owl, if you want to chat further you can pm me. Tomorrow will be a very difficlut day for you, but you are chosing to give your pet the gift of a peaceful passing. My thoughts and prayers are with you!
I am really upset, because we have had her my whole life.
.
And I'm really sorry. For me it was very hard to loose my dog, as it probly is to you, too. 
~*Prayers and Pixie Dust*~ 
