That being said, you need to be careful. Where most people trip up is in innocent conversations. The ones where you sit down at lunch and start discussing a possible idea with someone and the next thing you know they took it and sold it on their own. Here are some tips to help you avoid these types of situations.
Register Your Work: I know, I'm starting with finished scripts but this one bears repeating over and over again (because people think they can ignore it). Before you even think about sending your script out you need to register it with the Writer's Guild and the US Copyright Office. There will be people who tell you that you don't need to do this because US law establishes copyright the moment you create something. Don't listen to them. If you end up going to court someday do you want to base your case on "No way, I'm the one who wrote it. Not him!"? Or do you want to base it on "Here is my US Copyright, registered on this date."? We're talking $20 for the WGA and $35 for a copyright. There is no reason not file for this extra protection. (PS: If you make changes to your script, you need to file for another copyright. So don't register until you think you are absolutely ready)
Use The Google on The Internet Machine: Call this a leftover from my law enforcement days. If I am given the opportunity, I want to know everything about anything about any person I am going to sit down and talk business with. Done Deal Pro and IMDB Pro are your friends. Google is your friend. With these two simple tools you can find out where someone has worked, any deals they may have made, what circles they travel in and who they have worked with. That seems like a better idea than just blindly sending your scripts out to people just because they have "Manager" or "Producer" after their name.
Follow Your Instincts: When I was a young officer, I received some of the best advice I have ever heard. A veteran officer took me aside and said "When you are interviewing someone and the hair on the back of your neck starts to stand up, you are always right. Every time. It may not be the thing you are originally thinking, but there is a reason the person in front of you is setting off your alarms." It's not just advice for law enforcement. If you are sitting across from a manager or producer and something in your mind is saying "This isn't right", there's a reason for that. Listen to those warnings.
Silence is Golden: You can not register an idea, therefor you cannot protect an idea. So if you are sitting on the next great thing, an idea so great it sells itself in one line, then keep your mouth shut. It's as simple as that. Keep your mouth shut until such time you have that idea on paper in the form of a screenplay. Sure, there are exceptions to this rule, but they have to do with points 2 and 3 above. If Steven Spielberg is interested in what you are going to do next, that might be a good time to open up. Someone who barely registered in your research and you have some doubts? Keep your mouth shut (tactfully).
Know When It's Time to Walk Away: I'll give you an example for this one. Last October, Dustin and I sat down with a manager to talk about one of our scripts. From the first second of the meeting it just didn't feel right. To say the vibe we got from him was creepy would be an understatement. He didn't even want to talk about the movie. All he wanted to know was "What is the budget and who would star in it" (Questions not typically in the screenwriter's area of expertise). Before we even got a full idea of how to answer he said "Send it to me. Send me all of your other scripts, synopsis, or ideas as well. Include cast lists and budget break downs for all of them.... and make sure to sign our legal release" We smiled, nodded our heads and then walked away. Sometimes it's just not worth it.
Are these foolproof ideas? No. You show me something that's foolproof and I'll find you a better fool. But in this case, an ounce of prevention really does go a long way. Do what you can, suck it up, smile, get out there and have some fun.