If you disclose your idea without taking steps to protect it, you're giving it away.
An NDA sets out WHO you're disclosing your idea to, WHAT the subject of the nondisclosure is, and HOW you'll enforce it, i.e., what the remedies are for disclosure. Figure out these three things BEFORE you disclose your idea, and get an NDA drawn up.
WHO should be easy. A prospective business partner? An investor? Know who you're dealing with, and get their full contact information on the NDA.
WHAT is more difficult. You want to define the subject of the disclosure broadly enough to cover everything, but not so broadly that your description becomes meaningless. Remember that things in the "public domain" that are already known or obtainable from other sources won't be protected -- this is just for your specific idea.
Then make sure your NDA includes enforcement mechanisms: damages, injunctive relief, an attorney's fees clause to the prevailing party if there's a breach, etc.
A Written Contract
Yes, you can protect an idea. While it's true that copyrights only protect expressions of ideas, and trademarks only protect logos and slogans that tell consumers the source of goods or services, and patents only protect processes, it's not true that ideas aren't protectable. You can protect an idea submitted to someone if you make it very clear that you expect to be compensated for the value of your idea. When these "idea submission" cases are litigated, there is usually a written contract, but sometimes there is just a convincing paper trail -- such as confirming emails of verbal agreements -- that establishes that someone disclosed an idea with the expectation of being compensated. Verbal testimony can help, too, but your best defense against idea theft is getting your recipient to acknowledge that you expect to be paid for the value of your idea. If you can't prove you had an agreement, you won't be able to enforce your claim.