For writers, I think observation is vital. We should be constantly eavesdropping, making mental notes, observing details in our daily lives: like the way a squirrel rummages for food, the tone of a couple having a fight in a booth next to us, the awkward expression people wear when they're not being 100% honest with us (or with themselves!).
I actually developed an assignment for my students called an "Observation Paper": Choose a place (a park, library, restaurant, mall, bookstore, etc). Observe everything around you - the atmosphere, the smells, the sights, the sounds, the people. Use your senses to sharpen your focus.
Some of the best papers I've had from students come from this assignment. They love the freedom of it (I don't require a certain format - they can make a long list of detailed descriptions, or turn them into a poem or story, or write long paragraphs of description, whatever they want).
Often, our observations can end up in our stories/poetry. Little bits of actual conversations can be turned into character dialogue. An incident you observe can be the plot foundation of a future story. Someone's personality flaw can become the core of a main character.
Observation is a powerful tool that helps us sharpen our descriptive skills - and, can spice up our writing like almost nothing else can. As well, I think it injects a certain realism into the text that you can't get any other way...