Get the work coyrighted before you send it out, minimally you'll be able to prove claim to your own intellectual property. That gets your foot in the door, legally. This doesn't mean you need wait for the document to arrive before sending out your manuscripts or proposals. An article, applied for copyright, is held under that CR from the day it gets to the LOC. CRs are cheap...about $30 each for text. Send the work via fed ex or ups instead of the post office, have it sent for rapid delivery, pay the insurance, if desired, and arrange for tracking and signed receipts.

Earlier this year I caught a website with a glaringly obvious rewrite of "How to Winter Sow", it was paraphrased, but had so much of my text still in it that I wrote the sunofagun and expressed to him my desire that he either give credit to me and link to my site or the GW WS forum or pull the work. He balked. It was after I sent him a copy of his site, with his own words that the work was copyrighted by HIM, that he finally got the point that I meant business. 24hrs later he showed me an acceptable rewrite, he still won't give credit, but it doesn't matter as I can claim his plaguerism any time in the future.

Excellent advice. You might also want to show it to an attorney before you send it off.

Lovetogarden:
To a publisher it's not worth the hassle of getting a proposal and chapters from you, finding a writer to re-do all your research and writing, then do the rest of the publishing steps just to "steal" a book. What undoubtedly happened was that your idea was not as unique as you thought it was, and someone had already submitted a proposal that was similar.
If you think it's unlikely, I know two romance novelists who independently saw the same small news item and wrote books about it, had similar books published within weeks of each other ... and two others whose editors gave books the same title, releasing them within weeks of each other from different publishers.

"but from two of the publishers my idea shortly was printed by other authors."
How "shortly"? Publishing works slowly. If it was less than a year, they probably had a similar book already accepted on that subject in the pipeline. I had to reserve book printing runs for user manuals 3 months in advance for big runs - buying paper, setting type, proofing, etc. is not a fast process.

"These 'authors' even used my title!"
Titles are not copyrightable ... and EDITORS usually pick the title anyway, to suit the fad of the moment.

You can, for a small fee, copyright a year's worth of unpublished material at a whack. It's a good idea if you plan to submit these things.

TRUDI -
If yuo plan to nail the dude for copyright violations, THERE IS A TIME LIMIT after which you can't collect damages! And it's only a couple of years after you discover the infrimgement.

One way to "protect" your work is to make it as personal as possible in style, content, anecdotes, et al. It's kind of hard for another writer to copy the latter and claim them as unique experiences. For example, if you start your book by telling the reader that you inherited your garden because your uncle drowned in a butt of Malmsey, it would be tough for another writer to say the same thing happened to him or her.

Thanks Lazy, I know the regs....I am happy to allow anyone to do a cut and paste of any of my info for non-profit or educational purposes. All this guy had to do was ask....but that he couldn't do. And to then balk at doing a rewrite AFTER I nailed his butt was a bit much. I made my case clearly enough that he got the hint that I would protest legally and be able to win a plaguerization claim if he didn't rapidly make the change.

But is it worth the time? This guy's a flea with empty pockets and he's not going anywhere. If it was a big company I'd discuss it in a heart beat with AgLAW and let them advise.