Two things that are necessary to write are

1) Starting.

2) Continuing.

To write, you must at some point begin to write. As the song says, I've flown around the world in a plane, I've settled revolutions in Spain, but I can't get started with you. Starting is a lot harder than continuing. In fact, once you've started, it is hard not to continue for at least a little while. You can easily trick yourself into an hour or so.

So if you're not writing very much, you might want to analyze whether your problem is a starting problem (attack) or a continuing problem (sustain). If you have no starting problems, you are better off than most people. All you have to do is trick yourself into continuing for long enough. If you have no problem with sustain, but can't get started, then try to just begin by telling yourself that you'll only write for five minutes. Then, once the five minutes are up (you've already started!), you can just keep going. Or not. But at least you've practiced your starting.

If your problem is exclusively sustain--you begin to write frequently but cannot concentrate for long--you have several options. Analyze what is it that is interrupting you. The phone rings? Thoughts of self-doubt? (external or internal). If the cause is external, then try to eliminate it. Write after baby is in bed or before she wakes up. Turn off cell phone. Don't check email. If internal, then make a conscious effort not to listen to those voices telling you you can't do it. Make a list of excuses you make not to continue, and refute them one by one.