Take a step back to reflect. You're chronicling your life lessons, but what have you learned from them? Relay your intentions, desires, feelings of loss, feelings of joy, the wisdom you've gained, and other inner thoughts from time to time throughout the book. Taking a step back from the action of the story to reflect on what it all means is a good way to add depth to your autobiography.
If the purpose of your autobiography is to pass on your story to your heirs, consider including memorabilia (e.g. pictures, heirlooms, medals, mementos, letters, etc.) and putting your story in a scrapbook format. Of course, you may not be able to copy the memorabilia that accompanies your autobiography, so you still have to think about what you intend to do with your original work and other items, such as medals or bulky heirlooms.
Writing an autobiography for a high school or a college English class can help you gain a deeper sense of personal identity. An autobiography allows you to tell your story in a way that reveals truths about your values, goals and dreams. Though there's no exact science to writing an autobiography, you should include information about your background, major events that shaped who you are and any core themes in your life. Include specific examples to help readers understand your life history.