Those of you who know me know I am about as close to pure pantser as a writer who produces coherent stories can be. I often have no idea what's going to happen past the next chapter or so, and only a vague notion of how I'm going to get from there to the ending - if I know what the ending is supposed to be.
For those of us who plot by the seat of our pants (pantsers), it's a fairly common experience. It's also why plotters think pantsers can't carry a plot in a bucket. Of course, those of us who plot in detail (something I personally can't do) have nice detailed outlines and they know beforehand what's going to happen - so those fortunate souls probably look at pantsers like me with complete incomprehension anyway.
So how can a pantser plot? One option is to ask Pratchett how he does it (he's probably the most prominent pantser in the genre at the moment) - but he's probably not going to be able to tell you. See, the biggest difference between plotters and pantsers that I can see is that for plotters it's all up front in the conscious brain. Pantsers just about everything is subconscious until it needs to be made conscious (usually while you're writing it, sometimes later than that). I'll find myself dropping something in for no reason I can see - and then later it turns out to have been key foreshadowing.
Anyway, here are some of the things I do in lieu of having a carefully planned out plot. I try to have a very good idea what drives my characters, even when they're being recalcitrant. I look for at least one overriding need which they're going to try to meet. In Impaler, Draculea's strongest need is to protect his family - a need which is so strong he'll risk everything on the slim chance of success - not least because there is no other option.
Which is - for me at least - the key to plotting. Once I know what a character needs most, I arrange to remove the options they have of getting it to drive them into the method that will challenge them most. Impaler was easy - it was all historically there. For other books, I've used weather, other characters, anything environmental I can so that the only choice they've got is the one that leads to the end. Sometimes, if I have a character with a strong sense of duty I can use that instead - the character's own nature will push him or her into situations that force the plot.
Another technique is to borrow from the world of psychology. I hope you'll forgive the digression into what's only a short distance removed from pop psychology, because it does make a handy tool to figure out what that oh-so-irritating character will do next.
What I'm talking about is Maslow's Heirarchy of Needs, which is at its core the simplest commonsense. The most basic human needs are for food, safety, and shelter - not necessarily in that order. If those aren't satisfied, higher order needs will be ignored in favor of meeting the lower order needs. So, starve your character, and his most urgent priority will be food. Put him at immediate risk of his life, and he'll do his damndest to survive. Strand him in the middle of winter, and he'll focus on finding somewhere out of the weather. And so forth. You can drive a plot a long way by this method, throwing obstacles that force your main character to fulfill a basic need in a way that will make their real goal harder to achieve.
Another useful technique is to re-use the minor characters, letting them help or hinder the main as the need arises. Pratchett uses CMOT Dibbler - and his many avatars - this way. Dibbler is mostly a background noise character, but he's always there and can be relied on to complicate things, drop a useful or frustrating hint, and sell food items of dubious origin. Pratchett could have worked with anonymous food vendors, but with Dibbler, he doesn't need to, and he has quite a bit of layered information in the things Dibbler does.
All of these things can help, but there's no real substitute for understanding plot structure. The thing with pantsers is, we need to understand it at a conscious level as well as an instinctive one. I use that order because that's the order I learned plotting. I read so much I have a built in grasp of what plot structure should look like. The result is that I'll naturally spawn intricately layered plots without really understanding what the heck is going on until - in extreme cases - I've finished writing the book. This isn't a good thing - the Epic With Everything is 160k words, and was written while I was still in uber-stripped-down style with next to no setting. Properly cleaned up and expanded there's at least three novels crammed into it, all of them hopelessly intertwined. Maybe one day I'll get good enough to tease out the structure of the thing and make it work as separate novels.
By learning plot structure at a conscious level, I have a better idea what's happening, so I can consciously shape what I'm writing to improve the pacing and foreshadowing in what I do.
What are some other ways pantsers can improve plotting?