Keep your writing momentum

If you're like me (and if you are, I pity you), then your writing flow is full of stops and starts. Just when you get into the writing groove, the world (or worse, yourself) conspires to interrupt you and break this -

- hold on, bathroom break -

...

...

Where were we? Oh yeah.

- break this sublime streak of productivity. That's not good for your word count. Nor is it good for whatever you're writing, because you're going to lose your train of thought and risk going off on tangents that are totally irrelevant because that's what tangents do and...

(deep breath)

Sorry. Anyway.

There is help for us, my friends. There are ways to keep you in your seat and pounding happily (or unhappily, for some of you) away at the keyboard. And not all of them require cattle prods and electrified fencing. In fact, most are quite pleasant.
1. Write to a goal

This is the tip that seems to work best for me. Your goal can either be word count, a number of chapters, or a set time period. The point is to have something to shoot for: small manageable goals that you can repeat on a regular basis.
2. Psych yourself out

It's not just for athletes. Getting in some serious mental mojo can really get you pumped up to write. Meditation might work, if you're into that. But egging yourself on works too (just don't believe your own hype.)
3. Get a cheerleader

If cheering yourself on seems to weird for you, then you might consider having someone else be your cheerleader. This person will hold you accountable for the goals you set, and if they do their job right it can really push you to be productive - especially if there's a reward, like a round of drinks or something. Your cheerleader can be your spouse, your friend, or even a fellow/rival writer (for some people, it's competition that brings out the best in them.)
4. Remove unsafe distractions

Notice I said unsafe distractions. There's no way to completely remove yourself from distractions if you really can't concentrate. The key here is to know which distractions are the most debilitating to you - like the Internet, in my case, or noise for others. Then you can use safe distractions like music to shut out the outside world.
5. Move somewhere else

Most writers work at home, and the problem with working at home is that sooner or later a family member is going to ask you to do something. I go through this with Flossie sometimes, and I do admit that it's hard to say no. The solution? Remove yourself entirely and work somewhere else. The library. The coffee shop. Anywhere you can work comfortably and not be disturbed.