In an article in the London Times, novelist David Mitchell says he was terrified when he realized he had to write a book in the third-person. He says, "If you're an omniscient Godlike narrator and you can see everything from every angle and into the future, too, where on earth do you stop?"
To deal with this dilemma, he developed an imaginary thought spike-helmet: "I imagined there was a helmet mounted on the head of one character per chapter. The helmet has a spike in it that can go into that person's head and read that one person's thoughts, but nobody else's."
(There is lots of useful help for both fiction and non-fiction writers in my book,"Your Writing Coach," published by Nicholas Brealey and available from Amazon and other online and offline retailers.)