Life is full of waiting.
It's a fact: sometimes your life will be in a holding pattern. Waiting sneaks up and steals little parts of nearly every day, or eats large swaths of our lives in great gulps. It's waiting in lines, in doctor's offices, waiting for packages to arrive - but it's also waiting for the weather to change, waiting for good or bad news, for energy or happiness to return. It's so easy to become nothing but the anxious waiting for the next thing in our lives to begin. But in reality, waiting time is no different than other available time. We've all got to use this time productively, so that time spent waiting isn't wasted.
Read, read, read.
There are twenty-minute snatches during the day that are made up of waiting, but are often lost because it's not enough time to do something. The secret is that it's always enough time to read. Always, always have your book on you; or, do as I do and always have a Kindle when you're getting on the train to go somewhere. The people who have read thousands of books don't sit down with their tea and just the right armchair to read every time; they read everywhere and at any time. If you're waiting for anxious reasons, it's also very soothing to lose yourself in a book on a regular basis.
Make story notes
Just because you're stuck waiting for something doesn't mean your writing has to stop. Instead of trying to write fully, make small, quick notes about what should happen next in your story. Make a list of scenes to be written, or jot a note about why that character is acting the way he is. Later, when you're ready to write again, these notes will be invaluable for getting you started. And if for some reason you can't even make notes, use your writer's eye to make observations. There's always something to look at in greater detail, always something you can work to find just the right word for. It'll keep your writing muscles sharp.
If you can't write, prepare your space for writing
The anxious nature of some waiting can keep us from writing at all. That doesn't mean you can't prepare for the time when you can write again. Set up your writing space just the way you like it. Get the notebook you want. Dust. Clean. Assemble your favorite books. Respect the rituals of the creative endeavor, and the muse will respect you and return.
Don't fall into the trap of not thinking
The worst trap we can fall into while waiting is the trap of not thinking. We want the time to go by as quickly as possible, so we allow ourselves to sink into an uneasy gray funk, watching the minutes go by, not questioning or wondering or working. Don't let this happen. Letting time drool out of our lips is not making waiting time better! Keep yourself busy and stimulated, even if it means looking out a window and trying to think of names for all the colors you see. Do the things you wish you were doing while you're waiting, and the waiting won't seem like any sort of waiting at all.