I can't claim to know the finer points of law. But personally, anytime I am involved in anything that has intellectual property rights attached (Weather it be graphical, website design, writing, etc., etc.). If I intend to claim any portion (if not all) of the rights attached to the product, I make a hard copy of my work/involvement on disk (both External and HDD), sometimes I even add copyright date and explanation of what that copyright encompasses.
As far as ideas are concerned. If you're talking about a general idea like, "A group of Vampires attack a town, but one of the Vampires gets a conscience and fights back". There's not much for IP to attach to, because anyone might come up with that idea on their own. But if you had a product that went into detail of how the world worked, the characters involved, etc. like that. That might be a different story.
Take for instance Never-Never-Land from Peter Pan. Just by the name alone someone might assume that the concept added to JB's inspiration (I'm not claiming it did or inferring anything, just making an example). However, it's completely obvious that JB's Never-Never is completely his own creation. And any ties that anyone might be able to find that resemble Never-Never-Land and the Never-Never would be extremely loose.
And that is essentially what IP is. It protects you from your works/products being regurgitated. You can trademark certain things, etc. And if anyone is inspired by your work, they are allowed to be, but they can't use your base product. They can use the over-all concept, but they have to make every bit of their product as their own work and not tied to yours without permission.
However, if you have an idea that you think is wholly unique, then your best bet is to keep it to yourself or people that have signed DNC agreements. If it's that important to you. But understand that once it is out to the public, people might be inspired by it. Which would be good for you (The whole point is to inspire and entertain, IMHO). What you gain by keeping it to yourself is being the first to use the idea and staking your claim in history as being the creator.