It's tempting to take nibbles at different activities on your to-do list. If you actually want to be productive, you're better off dropping the multitasking and getting one thing done. In fact, there is a nearly-century old theory that explains why.

The Zeigarnik Effect is named after Russian psychologist Bluma Zeigarnik. In her most famous experiment, she found that waiters were more likely to remember incomplete or unfinished orders than fulfilled ones. It was so extreme that they often could not remember any details of the finished orders.

Her further research supported one simple idea: Completed tasks are rarely remembered, yet incomplete tasks dominate our minds.

The problem is that having several sort-of complete things on your to-do list will make you less productive, as it will require more brain power for you to actually focus. However, if you actually complete the items one-by-one, you are more likely to forget about the finished tasks and give more attention to the important remaining items.

Here are some ways to boost that productivity and fight the Zeigarnik Effect: