Ideas come at odd times and you need to be prepared. Carry a notebook with you everywhere you go. This can be a pocket-sized notepad, a cheap steno pad that fits in your purse, or one of those pretty decorated things that cost way too much money but make you look really cool when you're writing in it.
What the notebook looks like doesn't matter. That it exists and is within easy reach is critical.
Let's say your waiting for your turn in the dentist chair and you overhear a clever conversation between a mother and child. Get out your notebook and jot it down.
Or maybe you're at the park, and overwhelmed by the Spring-ness of the day. Write an in-depth description of what you're seeing, hearing, smelling, touching, tasting.
Or let's say you're struggling with a character and while mall-watching (you do go hang at the mall just to watch the people go by, right?), you see the perfect embodiment of a secondary, but critical, character. Whip out that notebook and write down the details—from the greasy long black hair and pierced left nostril to the holey Keds® sneakers on his feet.
If you don't keep a writer's notebook, start one today and practice using it. I'll be posting more uses for this notebook next Tuesday.