How does one prepare for the end of a pet's life?

Yesterday, 9/1, my 13 yo dog had an ultrasound which confirmed the worst of several possible diagnoses: she has stage IV hemangiosarcoma of the spleen with multiple macroscopic splenic and hepatic mets. There are questionable spots in her lungs also. It is too late for surgery, and her type of cancer responds poorly to chemo. I knew that she was getting older and her time would ultimately come, but she is still active, loves walks (a little shorter this year), and loves her meals. She also enjoys being close to me where ever I go, and chooses the door of the room I may be in for her spot to snooze. She currently has many more good days than bad. I would like to hear from EP members who have dealt with a pet's terminal illness: How does one prepare for the end of a pet's life? And how does one evaluate that point where the bad days out weight the good ?