I HATE that question! For interviews, I tried to break it down into categories mentally, and give them one or two sentences about each.
How I grew up/where I am from (e.g. I am from Southern Virginia and grew up on a farm)
My hobbies (e.g. I love to hike and I breed reptiles and I am teaching myself Portuguese - they want to see if you are well rounded)
My school majors and experiences ( e.g. I was a Biochem major, I worked at the Naval Research Lab doing chemistry, I worked in a histopath lab, etc and say why you liked those experiences)
How my current interests mesh with vet school ( e.g. I have a great interest in research/diagnostic pathology and infectious disease and would like to pursue that in the veterinary field)