We just went throught that this past weekend as well. For us it was our 14 year old Golden Retreiver. Last Sunday everything was great and he was chasing balls and running around like always. Then on Monday he could not stand up so we took him to the vet and he gave us some Prednisone (This happened last fall and we thought we were going to put him down then but then he made a miraculous recovery). Well, this time he would not eat anything and kept getting sick and couldn't move. It was very heartbreaking. Took him to the vet again on Friday and they did some bloodtest (to rule out Lymes disease and see what was going on). Turned out his liver and kidneys were failing also. So, we had to put him down this past Saturday too. I thought we were prepared for this but turned out to be harder than I thought. My DD9 is having the hardest time with it.
We know we will get another Golden but I know I will be comparing them all to Jake. He was so good. Stayed in the yard (without a fence). Rarely barked, never chewed on things that weren't his and always had a dopey look on his face. Would bring you a tennis ball and put it in your hand to throw.
We still have our other dog, which is a 15 year old Pekingese. He is partially blind, deaf and gets lost in our yard. I'm sure he is missing his buddy too...