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.
Create an outline or a graphic organizer of the major topics and ideas you want to cover. For example, you might make a time-order chart -- a chart with event blocks to organize events chronologically. Or, you might draw a cluster web -- a diagram with connected circles -- to divide your autobiography into sections, such as one for relevant background information, one for life-changing events, one for special memories and one for goals and dreams. Focus on reasons why specific events, situations and circumstances were important to you and how they made you feel.