At Concurring Opinions, The Truth about Multitasking:

I’ve been of two minds about multitasking for some time. But growing evidence is suggesting that the very concept is a myth:

Dr. Edward Hallowell, a Massachusetts-based psychiatrist who specializes in the treatment of attention deficit/hyperactivity disorder and has written a book with the self-explanatory title CrazyBusy, has been offering therapies to combat extreme multitasking for years; in his book he calls multitasking a “mythical activity in which people believe they can perform two or more tasks simultaneously.” In a 2005 article, he described a new condition, “Attention Deficit Trait,” which he claims is rampant in the business world. ADT is “purely a response to the hyperkinetic environment in which we live,” writes Hallowell, and its hallmark symptoms mimic those of ADD. “Never in history has the human brain been asked to track so many data points,” Hallowell argues, and this challenge “can be controlled only by creatively engineering one’s environment and one’s emotional and physical health.” Limiting multitasking is essential.

Walter Kirn concurs: "Neuroscience is confirming what we all suspect: Multitasking is dumbing us down and driving us crazy."

I’m fond of the "Getting Things Done" method and its permutations. Get "stuff" out of your head into a simple form on paper/computer. Then do one thing at a time, and try to do it until it’s done.