I like to keep the space under the bed clear but sometimes because of space or lack thereof, its necessary. So choosing how to use that space is important. You want to maximize that space just like any other in the house. And you want it to be neat and organized.
My tip is to get some plastic storage containers. They have some that are long and not too tall and with wheels. My suggestions is to store frequently used sheet sets for the bed. Or extra blankets and pillows. One of my clients put her sweaters under there. She only wore sweaters certain times of the year so that freed up her small closet for her everyday wear.
Another tip is that under the bed can be a great space for hiding birthday or holiday presents too. Put them in the middle of the space,out of easy reach.