Concentrate on the task at hand. One reason is that “when forced to multitask, the overloaded brain shifts its processing from the hippocampus (responsible for memory) to the striatum (responsible for rote tasks), making it hard to learn a task or even recall what you’ve been doing once you’re done.” This disheartening glimpse from neuroscience comes from Julian Dibbell, ed., The Best Technology Writing 2010 (Yale Univ. 2010) at 189 (by Sam Anderson).
Our brains process serially, not in parallel. If we want to think hard, we need to think solo. Put differently, “When you think you’re doing two things at once, you’re almost always just switching rapidly between them, leaking a little mental efficiency with every switch.” (at 191)
Although I succumb to distractions, I have bashed mental juggling on this blog (See my post of June 16, 2010: internet distractions and some tools to cope; Sept. 20, 2010: myths about concurrent thinking; Aug. 26, 2009: trying to do too many things at once with 8 references and 1 metapost.).