Definition: Memoir is about an incident in my life, not my whole life which is an autobiography. Also, memoir is “softer” than autobiography in that it tends to be more about impressions regarding events than hardcore reportage of the details of the events themselves.
Educate your reader: Don’t assume, if you’re writing about some legal matter, for example, that your readers will find it boring if you divulge enough to let us in on it, but not too much that it turns into a college course.
Don’t depress your reader: Even the saddest story has its bright spots. Write with some lighthearted relief among the heavier moments and even some humor if and when appropriate, remembering that often the best humor is directed at yourself.
Tell me a story: Memoir is nonfiction; but always remember you’re still telling a story, with a beginning, middle, and an end. How were things to begin with? Then what happened to change them? Then what did you do, and how did it go? How did it all turn out? Did you learn anything?
Not therapy: Even if it feels like therapy to write about a tough time in your life, don’t let that come through to your reader. This is a memoir not a psychiatrist’s couch. Hold back raw anger. Work that out in a journal before you start writing your memoir. Just tell us what happened and allow us the intelligence to figure things out.
Not me against them: It’s easy to think back on an event and see ourselves as the only reasonable ones present at the time (me good, them bad), but be realistic and honest (dare I say, mature?) in your telling of the story, including those times you wish you’d acted differently. This adds depth not only to the story but also to you, the main character in it.
Be factually correct: This is true with all writing. Don’t insult your reader by not checking factual info before publishing. Was that town, where my car skidded on the ice and I broke my leg and the orthopedist turned out to be my future husband, in California or Nevada? Use the internet to look it up if you are not sure.
Be tough: It’s not easy to go back over past events that were anything less than joyful in the experience. Be sure you’re willing to do so, or the story may come out stilted. I think it was Mark Twain who said if you’re not crying while you’re writing, your reader won’t cry when he reads it (he said the same about laughing, too).