If you are a reporter staring down the barrel of a libel complaint, there's no better defense than proving that the objectionable speech is in fact the truth. The truth is the best shield that a journalist has. It can be substantial truth -- a few minor inaccuracies won't derail a report that is largely correct. But it often must be the whole truth -- it can't omit true and important facts that create a false impression about someone. Also remember, the truth is also no defense for false light claims. My page explaining false light can be seen here.
If You Are a Reporter on Deadline Worried About Libel:
There is an excellent website put out by the University of Texas system you can go to if you want to "test" to see if your work could be libelous. It is a way to analyze your work to see if it is libel prone, and can be found here. There is also a useful libel FAQ that the Media Law Resource Center provides on its website.
As long as the reports are fair and accurate, they can for the most part report defamatory language made during official proceedings. A senate session is an official proceeding. An off-duty FBI agent making remarks in a smoke-filled bar to a reporter does not qualify.
An opinion is protected so long as it cannot be proven as a testable, provable fact. This is why the Milkovich case was so important, because the Court determined that a columnist expressed an opinion that could be tested -- that he believed Milkovich perjured. There's a big difference between saying you think someone is an adulterer and saying that person is annoying. In one instance you need to prove someone committed adultery to enjoy protection from a libel suit, in another you are simply stating an opinion that you hold yourself
Believe it or not, you can defame someone and be completely protected under U.S. law. The defamatory words of public officials and political candidates acting as such are protected. Oddly enough, you can also defame someone with their consent. There is also such a thing as a "libel-proof" figure -- someone who has a reputation which is so execrable that they may be ineligible to bring a libel claim -- you can't damage a reputation that is already utterly destroyed. This can be a tricky concept, however, as a person who has no reputation in one area may have a sterling reputation in another. For example, if someone implied that a rock star who had repeatedly checked in to a rehab clinic were a habitual substance abuser, that may pass muster. But if someone implied without proof that the same rock star stole someone's copyrighted music, that could be a problem.