Here's a simple challenge: For one month, write something every day. It doesn't have to be a story, it can be a poem, a description of your cat sleeping under the sewing machine, or just a discussion of a dream. The real mission here is to do something regularly, even if it is just a quick haiku:
Writing this haiku
Satisfies today's task for
my daily challenge
There. Done. Simple as that (let's pretend I wrote that on a different day than this post). This exercise, if done for an entire month, helps develop writing as a habit and not an exception, and flexes the creative muscles. I often relate regular writing to regular exercise. The first time you walk a couple of miles on the treadmill it will be exhausting, but if you do it for a month you will notice a difference. You might even want to add an extra mile on your walking sessions. The point is, you will have made it through the awkward adjustment part, and have entered the growth phase.
Here's an exercise I enjoy for NaNoWriMo month. Every week, I try writing something I would never have taken on before. I'd write about an ancient memory as seen through my adult eyes. Look at the world through my parents' perspective and write their viewpoints. Recently, I wrote about my personal recollections from having a seizure when I was twenty - a very scary event that forced me to really stretch my writing muscles. That's the purpose of these kinds of exercises - not to build endurance, but to expand perspective and add to our our creative toolbox. None of this has to be good, interesting, or worthy of sharing - it just has to be done. And if it helps you as a writer, why not do it?
And, as a much simpler exercise for those times when you just can't write, explore the rest of the creative world. Read a new book. Go to an art gallery or a museum. Search Google Images for Renoir paintings, and just admire them. Sometimes, when we just can't be creative, we can still participate in the creative world, and that's something as well.
Happy NaNoWriMo month, and keep on writing, or at least keep on creating.