It is always tough to know if you’re doing the right thing when you put your hard work or brilliant ideas out there, where other creative people are always looking for their next golden egg. I’ve been getting lots of questions about this topic and there is only one really good answer.
Register your work with the Writers Guild(s). The most commonly used one is the WGA West. You can choose the one nearest. I register with many of our Canadian Guild too. I’ve also used Copyright through the Canadian government.
Some other good practices are to use watermarks and samples. There are a lot of watermark apps and programs out there. I use Watermark PDF Pro.
Don’t send out your complete script, screenplay, manuscript, etc.… without having a contract in place. There is a sample contract on the contract page.