You probably have a private space to write, somewhere quiet, a favourite lap-top or pen and notebook, a particular brand of coffee you drink whilst writing, a favoured plotting process, a particular time of day for writing, a particular numbers of hours for doing it, a word count to adhere to.
You can already see how easy it would be to upset the above writing routine.
We all have mechanisms to get us in the mood for writing, we'd be foolish not to. Real-life crowds in our writing time and space and if we're not careful will squeeze it to nothing. We have to be vigilant and keep our writing time and tools sacred, but it's easy for these support mechanisms to become crutches that we can't live without, and in the extreme actually inhibit our writing.
The key is to focus on flexibility, not rigid systems – here are some things to look out for and ideas on achieving a more flexible approach to the craft.
Noise and distractions are anathema for writing, but if you demand complete silence that's going to limit the opportunities and places for doing it. Don't let noise get in your way, invest in some headphones and some un-invasive music – even better go somewhere noisier so it all becomes background and less intrusive. I spent this lunchtime writing in a café – quite possibly the noisiest place I've ever worked but due to the generality of the noise it faded to nothing once immersed in my writing.
Learn how to write anywhere – writing isn't something you do when you have the time in the right place, it's something you are.
Don't just have one favoured tool you have to do your writing on, have a whole suite of weapons at your disposable – laptop, netbook, notebook (both kinds) etc. Carry your work on an USB stick so you can work on any computer anywhere in the world, carry a laptop/netbook/notebook (both kinds) with you wherever you go so if the car breaks down you can crack on with the WIP. There can be no excuses.
You can get into a heated debate about pants vs plots but real writers do both. Brick-walled whilst pantsing through your draft? Get the damned index cards out and plot. The sterility of plotting driving you insane? Then just pants the thing. Process is a means to the end, and shouldn't be the end itself.
I used to think that if I didn't have at least four hours of writing time it was pointless starting. I then used to think that if I didn't have at least an hour's worth of writing time it was pointless starting. Now if I get ten minutes, I write. Anytime. Anywhere.
Don't limit yourself to your WIP – sometimes you can get exhausted with it or have some other ideas that need exercising – write a short story, start a plan for WIP2, work on a blog-post, your synopsis or query letter. Failing all that, do some writing exercises. If you really feel you can't do anything then just write down ten observations of the place you find yourself in. It still counts.
This is what we are. This is what we do.