One way to prevent that from happening is of course, to limit who you show your work with. However another way is to take the manuscript put in a sealed SASE and send it to someone out of state and ask them to mail back your manuscript in the sealed envelope. The postmark will show the date so you can prove when you were working on it.
I will say though that there are a lot of copycat books out there. There are only so many storylines that a lot of books are similar. Once the Harry Potter books took off the market was flooded with similar stories, the same way with Twilight all of a sudden tons of vampire TV show and what not (actually some of the shows were way before the Twilight book. The next thing was Hunger Games and I'm sure that story was sparked by many of the fears people have about the way the US is changing over the years. I have read tons of challenge stories with those kinds of plots.
I'm so sorry that this happened to you and wish I could offer you more help. I know many of the big name companies like to settle out of court to avoid the publicity. If you have found several lawyers who agree with you, you may want to keep looking and see if you can find one who would be willing to at least write a letter. Of course you would have to have some kind of proof that you were working on yours way before the other author was working on hers. It takes years to get a good book together so you would need proof from at least 3 years if not more from when the book was released.
Thanks for sharing your story though. It is a good reminder to be careful. I found thias article and thought you might find it helpful http://lorelle.wordpress.com/2006/04/10 ... r-content/