Okay, sometimes you have to multi-task. Your job may be operational like mine. So what is better than multi-tasking? Chunking is better.
Chunking is the concept of breaking up your day into larger chunks instead of reacting to each "emergency". The more chunks of time you can devote to specific tasks, the fewer start-up moments you will have. Since you won't be spending as much time in start-up moments, you will have more time and you will get more done. As a bonus, since you will be able to focus on the single task at hand, you will do it better.