Someone found my blog, according to my blog stats, by searching for how pantsters make the most of their writing time. I have a few tips scattered here and there on my blog, but not an explicit post on the subject. So, I thought I'd toss out an answer.
Treat it like a second job.
When you're at work, you have expectations, you work on fulfilling them while you're on the clock, and you concentrate on that work (ideally). The best writing productivity times I've had are when I get rid of pesky distractions, tune out everything but my word processing program (or pages for handwriting), disconnect from the internet, and just write. It inevitably starts off rocky, for the first fifteen minutes or so, as I fight the urge to reconnect to the wireless router, or itch to click on a game. I'll sometimes have a temporary writer's block, which I get through with the methods I talk about in the post I just linked.
For the writing time I've set aside, I treat it like I'm being paid to do it, and like I'm stealing from the company (in this case, myself) if I fritter away that time.
Set a goal.
In my writing group, we set goals at the end of each meeting. Ideally, those goals are something we'll have to push ourselves to meet, but no one is grading goals, or punishing anyone for not meeting them. They're a personal watermark on which to judge whether we chipped away at what we wanted to do. It's something to keep in the back of our minds while we're doling out our time expenditures throughout the month.
Whether it's a goal for the week, the month, the year, or for the hour, having a goal in mind makes it clear that you're making progress during your designated writing time. Also, it should, ideally, help you remember just what you're working toward in your "second job."
Take a break.
As I outlined in this post, it's important to recharge the mental batteries when you're writing. Anything and everything you experience can and should end up in what you write. An overheard conversation, the way light reflects off a puddle, or the sound of rain on a car roof, can all turn out to be the key that unlocks a passage that isn't working for you. I don't know about other writers, but my inspiration comes from really odd places. If I didn't give myself that time to not be writing, I wouldn't have those experiences with which to enrich what I'm working on.
I've heard a lot of writers say that some of their best ideas have come in the middle of a walk. If you can, give it a try.
Your mileage is going to vary wildly on this one, because not everyone is a lifelong daydreamer, and it takes practice to pay just enough attention to the world around you. But, when I'm doing something boring or rote, I slip into the heads of my characters to figure out what they'd say in a knot of dialogue I'm trying to untangle. Or, I ponder what might pull my characters in the direction I need them to go.
I have to be careful to still pay attention to the world around me when I do this, and I have to pick the right time and place. But, without a little boredom in my life, I wouldn't get half as much writing done as I do.
This step should come naturally to my fellow pantsters. Isn't most of your writing just an extended game of "what if?" on paper, anyway?
Make a habit of it.
However often you choose to write, do it regularly. It doesn't have to be at the same time of day, though I've heard that getting up an hour earlier to write during the quietest time of the day has good results for those of you who can pry your eyes open to do it. (I'm a night owl who has to wrestle her conscious mind into slumber every night, so, not so much.) It doesn't even have to be daily, though I've also heard about good results from challenges to write consecutively every day as a goal. What's important is that you do make it a priority, and set aside that time, and keep setting it aside.
If you're too tired and distracted and cranky to write, don't try it. If you've been banging your head against a blank screen for more than fifteen minutes, walk away. If writing is a consistently frustrating exercise, you need to get away from it, and figure out why that is. If the problem is distractions in the form of twitter and facebook and blogs, then it's time to disconnect so that you can get your writing done. If the problem is environmental, find somewhere else to write, or consider handwriting your words for the day and typing them up amongst your distractions. If it's writer's block and you've tried all the remedies you can find, then try taking a long walk or otherwise doing something active where you can't write anything down until you get back. Take a nap, if you need it.
If you consistently have some excuse or another to not write, you might have a problem. But, forcing yourself to write when it's making you more and more miserable won't solve that problem. Figure out what's going on, call out sick that day (figuratively), and fix it.
In the end, it boils down to being accountable to yourself for getting those words onto the page. Only you know what motivates you, so you're the only one who knows for sure what will work and what won't. But, if you want to write, you'll write. The rest is just details.