In her series of books "The Artist's Way" and "The Right to Write," Julia Cameron instructs her readers to write three pages of prose every morning. These three pages can be about anything and do not have to be grammatically correct. No one is going to read them but the writer. The idea behind daily pages is to open your mind and to develop a regular writing practice.
In "Writing Down the Bones," Natalie Goldberg recommends "visualizing a place that you really love, be there, see the details. Now write about it." Write everything about the place that you can remember. The way it smells, the sounds, the colors, the way the wind blows through the trees and the kind of people who are often there. Write it in a way that helps readers feel like they are there as well. This can be a great practice if you have trouble writing setting.
Write about a favorite childhood memory. Is there anything you'd like to change about that memory? Are there specific details that seem dull or unimportant? Use your imagination to expand on your memory, adding fantastical creatures and new ideas. Write about what would happen if that event in your childhood happened to you now.
Walking meditation is another practice recommended by writers like Julia Cameron. Put on a comfortable pair of writing shoes and walk around your neighborhood without a sense of direction and purpose. Allow your mind to wander and allow your eyes to observe the world around you. When you get back home, sit down immediately and write about your walk.
What would happen if something out of the ordinary occurred, for example, if it snowed in July or was 100 degrees in Antarctica? Writing about something abstract and impossible is a great way to inspire your imagination and improve your writing--even if you are writing something expository.