There are two kinds of people: People who are bad at multitasking, and people who don’t realize that they are bad at multitasking. This is not an opinion; there is research to back it up. Here are a few links.

The Cognitive Costs of Multitasking

The Myth of Multitasking

Multitasking May Hurt Your Performance, But It Makes You Feel Better

So what to do? I have a few thoughts:

  1. Realizing that humans aren’t good at multitasking is an important first step. 
  2. If you have multiple tasks to execute, model your behavior on how single-processor machines implement multitasking.
    1. Realize that context switches are expensive.
    2. Divide your work up into easily-consumed chunks.
    3. Pick a single “work chunk” from the list and execute it from start to finish.
    4. Continue picking chunks from the list and completing them.
    5. If interrupted, come back and finish off the work chunk you were on before moving on to another.The goal is to have a pile of completed chunks, a pile of un-started chunks, and exactly one chunk in progress.