I didn't do anything special to commemorate my cat Chip. I have a lot of pictures of her though. My parents have helped me remember good things about her.
My new cat is all black and has a disability. He was shot when he was a kitten and his right front leg was badly damaged. He's not in any pain, but he limps significantly because his right front leg is about an inch shorter than his left front leg. It doesn't slow him down - he handles the stairs with no problem and runs around fine.