The %run magic provided by IPython not only supports the execution of regular Python scripts, it also runs Jupyter Notebook files (.ipynb).
The run_demo.ipynb notebook below uses %run to execute all of the cells in reusable_stuff.ipynb. Once it does, both globals defined in reusable_stuff are available in run_demo for use.
You can maintain a handy notebook of useful recipes that you can than %run to reuse in other notebooks. Just remember that this setup can decrease the reproducibility of your work unless you provide your recipe notebook alongside any notebook that uses it when you share.