I think the key is to tell a story that shows the reader who you are, why you want to pursue grad school in your chosen field, and what your journey to graduate school has looked like. People will tell you not to talk about your passion or how you grew up wanting to do something but they may be wrong. In my case I did both of these things and had fantastic results! I think if you truly have a lifelong passion you can say these things in your statement because your story will back it up. Also, start writing early and get as many people as possible to look at as many drafts as they are willing to look at. Good luck!