As much as we don't want it to, it's going to happen. Whether it's by accident, disease or old age (hopefully the last of those), it will happen. You can prepare in advance by deciding what you will do with your pet's body. Creamation? Burial? Pet cemetery? When I lost my hamster, long, long ago, we had a special burial ceremony for him. At the time, I was fascinated with ancient Egypt, so I buried him with a few of his favourite things. I couldn't build a pyramid, but I would have! For children, I think having a special ceremony is a good way to say good-bye. Customize it, and let them do what they want to do (within reason, of course) to get closure. It's good for adults to do something special as a send-off as well. I know I was so distraught over the hit-and-run death of my first cat that I couldn't even deal with the cremation, and let her be cremated along with the other cats at the hospital, and never knew what became of the ashes. I've written a few brief paragraphs about pet cemeteries and pet urns elsewhere on Myhealthypets.net that you might want to read if those are things you are interested in.
If your pet is aging or ill, and you already know that you will want another pet, it might be a good idea to get one while your first pet is still alive. It depends on your pet though, so use some judgement! But a new puppy or kitten will not only provide a distraction for your pet, but it can ease the grief of a death and fill the silence left by the loss. Whenever I've experienced a loss in my life, the presence of my cat or my dog has always eased the pain and given me something to care about.
There are a few comforting poems for the loss of pets, one of which really comforted me when I lost my first cat. I can';t seem to find it anywhere now, but as soon as I do I will post it here.