Something that I like to do is jot down brief sketches from time to time. I will spend fifteen minutes just writing a character sketch about one of the multitudes of people that live on the fringes of my brain. This character is not intended for use in any particular project that I'm working on--it's merely a practice for me at coming up with characters. And, of course, if I need a character later on, it can come in handy! Especially for providing depth to tertiary characters without derailing yourself from your current project. I often include a snippet of a scene that places the character in context, giving me a chance to see how the character acts. Sometimes this even leads me down the road of a new idea for a novel or story!
Another kind of sketch that I like is to try to capture pictures. I'm sure we've all had moments where our breath catches at the unique beauty (or horror) of something that we are observing. At least for me, I want to share that with people, but this is no easy task. Can you really explain the curve of that mountain fading into the distance? With practice, perhaps! I keep a file on my computer where I will type up a sentence or two and try to capture a particular moment in time. This is something that is very difficult for me--my writing tends to be terse and action-oriented--and so I must practice.
Here is an example about the view I had of downtown when I got to work the other morning:
The city rises into the sky, the highest buildings piercing into the bottom of a heavy gray cloud. Their tops are lost in the gloom, but between them rays of sunlight find a way to peek through, lancing through the buildings from some distant place where clouds do not hold sway.
Will this ever be useful in one of my projects? I don't know. I don't tend to write many locales that involve skyscrapers! But even so, I think it is useful to develop my skills in passing along inspiring, vivid scenes to my readers.