Interruptions. When you're a writer, you know these happen. A phone rings. Somebody opens your office door, just to say hello. You have to pee.
You can head off these interruptions with a little planning. Turn your ringer off. Put a Do Not Disturb sign on your door. And, well... You HAVE to pee. Just lay off on the coffee. (Just a bit.)
But even when you have the best time to write, and everybody's leaving you alone, you can still get interrupted, by yourself. Yes, some of these are not really interruptions, but excuses (I need to check my email; Did someone just like my kitty picture on Facebook?; Maybe I should write a shopping list now so it will be ready later...).
Stop that. (You know these are pep talks for myself, don't take it too hard.)
A worse type of interruption is when you you have so many ideas that need to get out of your head that the writing is flowing almost too fast. In other words, you want to get the WHOLE idea out so that you won't forget it later [insert something about needing to get the jist of the plot down but that you really need to get to the end]. It's like your own mind is walking into your writing space demanding attention. "Hold on! I'll get to you later," you tell your mind, and set the smidgen of the idea aside in some little brackets. This way, when you have the time to really flesh it out later, you know where it goes. This came from bookbaby blog. I've used something similar, but the brackets containing some good information to help trigger your memory later is better than what I was doing.