Always carry a notebook and a pen.
"Everybody walks past a thousand story ideas every day. The good writers are the ones who see five or six of them. Most people don’t see any."
~
Orson Scott Card.
You really never know when inspiration will hit, however when it does you better be ready to write it down or it will be lost to you forever. It is of vital importance that you carry something, anything with you to record those moments when the muse whispers in your ear.
I can't tell you how many times I've been walking down the street, or driving on the highway, or eavesdropping on a conversation and had the thought strike me; say what a good idea for a story. Then by the time I got home and set in front of the computer it was gone. It didn't take me long to download a quick-memo app to my phone and to put a notebook and a pen in each car. Once I'd figured that out I now have literally thousands of story ideas to pull from.
Not all of your ideas will come to full fruition, most will remain simply a thought on a page. A precious few however will be played out in a short story or novel and you will have an abundant garden to harvest from when you need it.
I hope that you find this useful, and thank you for taking the time to read it.