As my boss Jane Friedman once said, there are three reasons a memoir will sell.
1. Celebrity. If you're a famous politician or actor or business guru, you can sell a memoir.
2. You've Had an Incredible Experience. Think of the girl surfer who got her arm bitten off by a shark but continued to surf. The child soldier in Africa who lived to tell the tale. The guy who decided to retire at 28 and move to a retirement community in Florida. The guy who lived "biblically" for an entire year.
3. The Writing is Incredible. I'm guessing that most people out there fall under this third category.
The thing is: I've traveled from one end of the country to the other and met a lot of great writers. The problem is: Everyone's writing memoir. That's OK, because there are great stories waiting to be told and a lot of literary agents who want the category.
But because you're going up against so many other submissions, writers need every edge they can get. That's why I say write a book proposal and submit it with your pages/query. It doesn't have to a super-detailed or long proposal. Anything will help.
Discuss target audiences. Discuss competing books. Talk about a basic marketing plan. Use the business side of your brain. And to talk about competing books for a moment (usually a section called something like "comparative analysis"), you don't just have to list the most immediate books. So if you're writing a memoir about taking care of a child with down syndrome, similar titles in the bookstore are not limited solely to stories that are memoirs about caring for someone with down syndrome. Competing titles include books about caregiving, informational nonfiction books about down syndrome, memoirs of single parents (if that's the case), and so on. Think about it from several angles.