Posting a WIP is definitely a balancing act. The first one I posted here on LJ, A Hidden Magic, dried up on me partway through and was "on hiatus" for over a year. I hated that -- while it was just hanging there un-worked-on, I felt like shit whenever I thought of it, which of course wasn't exactly conducive to making me want to work on it again. I finally did whip it into shape and I completed it before I got back to posting, but I swore I'd never post a long story as I wrote it again. Even though it was finished, I noticed that some people who'd been reading originally weren't reading (or at least weren't commenting) when I posted the last handful of chunks, including at least a few who still had me friended. That's frustrating but when it's been that long, it's not really surprising that some people don't remember the story and don't want to reread the whole thing to get back into it. :/
So, of course, along comes a challenge that a friend of mine requested [tickles Amy] and she asked please (if it ended up a longer story) could the first chapter go up on her birthday? It spawned a plot bunny and I really wanted to write it, but I knew it was going to be long and said birthday was looming. Whaddaya do? :) So I've got another WIP again, although I've been posting steadily. I slowed down from three chapters a week to one, but I'm still posting, and one or two more should wrap it up, thank whomever. And after that I'm never going to post a WIP while I write it again. [duck]
Because as a writer, I have this constant fear in the back of my mind that I won't be able to do it. That something will go SPROING! and the drive shaft will crack and the belts will be flapping all over the place and the wheels will fall off and the damn thing will just sit there smoking and spitting the occasional spark while my readers are going, "Yeah? Well? You're gonna fix it by Friday, right?" and all I've got is a screwdriver and I don't even know where to start.
I learned a lot about how to fix that sort of problem and keep going when I did NaNo last year. My forebrain is pretty sure that I'll never have a really long hiatus like what happened with Hidden Magic happen to me again, even if I did make a habit of posting WIPs which I swear I won't. [cough] But that doesn't make it any less scary to my hindbrain. It's like that dream everyone has of walking into a class where you haven't attended in a month and it's exam day and you have No Clue Whatsoever what's going on. It's the same fear, really -- being expected to perform in public and being completely unable to do so. I don't think there's any logical argument one can make to banish that fear, although if I'm mistaken I'd love to hear about it. :)
For me, though, the easiest solution is to just finish before I start posting. There's no reason for me to post while I write, really. I don't change things to suit readers, I don't take reader requests or decide not to do something a certain way because some people reading are going, "Well, I hope that doesn't happen 'cause that'd suck!" Not that my readers are the kind of people who'd be that rude, but you know, just in general, that's not how I write. I love hearing what people think about my writing, and seeing people speculate about what's going to happen or what the answer to a puzzle might be, but I don't change anything on the fly. So it'd be just as much fun to watch folks reading along if I were already finished, and a lot less stressful for me -- everyone wins that way.
As a reader I find WIPs which go on hiatus frustrating and occasionally I'll wander off and not come back. But sometimes I'll see that a story I read the first twenty-six chapters of three years ago has been OMGFinished!! and I'll bounce over and read the whole thing from the beginning, beaming the entire time. It really depends on the story. If it was good enough (to me -- everyone's mileage varies here) that I still remembered it fondly after the hiatus, then I'll pick it up and read it again and be very happy.
The first example that comes to mind for this is Elanor's Resonances of Middle Earth, which is a sequel to Mired in Middle Earth in case you want to read them and I strongly recommend you do. Briefly, the Fellowship actors (except for John, and there's a plot point there) get transported to Middle Earth during filming. They need to go through with carrying out the quest, and knowing what's coming doesn't always help and is in fact fucking scary in places. This is a great character piece and I love reading this to watch how differently the experience impacts each character and how they cope. Resonances is a continuation after they return to Earth, showing how they all cope with some major post-traumatic stress, because that was not a fun quest. When I first got into the fandom I was wandering around web pages and archives, guided by Google and the occasional recs list I was lucky enough to trip over; I didn't hear about LJ until later. I read Mired in Middle Earth back then, and found the sequel and dove right into it, only to find that it just... stopped, twenty-some chapters in. And it sat there for, what, almost three years? :( I'd honestly given up on ever seeing it finished, so when it finally was, I was ecstatic. It was a story I'd been reccing all along, despite its being an (apparently) abandoned WIP, because I'd enjoyed it that much. I've never read another story like it and I as well as a number of other readers were very happy to pick right up again when the writer came back to it three years later. Not all stories have that kind of staying power, though. Most stories have a much lower brain-velcro factor.
So if I see Chapter Fifty-Four of "Some Unmemorable Title" go by on my Flist and I think, "Wait, what's that? Have I seen that? Was I reading that?" and I can't remember, then it's iffy. If the summary sounds interesting and if there's a link to the previous chapter in the header, then I'll click on that and skim through it and see if it sets off any recognition bells. If there's no link but the summary sounds really interesting, then I might click on Chapter Fifty-Four and see if anything in the first screen or so jars a memory. If I go, "Oh, yeah, that one! Cool!" then I'll settle down and happily read.
If I still don't remember it, though, or remember only fuzzily, but that first screen or so sounds really interesting, then I'll look for a way to get to Chapter One. If there's a link directly to Chapter One, or to an index page for that story, something like that, then cool. If not, then I'll look for tags and see if there's one for that story title. If there're neither links nor tags, then we're back to, "Do I really want to read this?" balanced against how annoying it's going to be to dig up all the previous chapters. If not, then I wander off to the next thing on my Flist. If so, then I'll go looking for either a general fiction index page, or for memories of this particular story's chapters.
[And note that most of this also applies to current WIPs. In my fandom, and others I assume, there are a lot of ongoing stories with the same pairing and similar gimmicks, and some writers' summaries offer little or no information which would help a reader with a sieve-like brain and/or a very busy Flist recognize or remember that particular story. If it's been more than a few days since I read the previous part, much less a week or two weeks or a month, and I don't remember the title (titles are often pretty non-descriptive as well) and the summary doesn't ring any bells and the header doesn't spark my interest all by itself, then I go through the same process described above. And if I don't remember and the writer's not willing to help me out, I might well end up wandering away even if I was reading up to that point. And current WIPs have the added disadvantage that I probably do remember the title, since I've seen it go by on my Flist ((N-1) * #FicCommunities) times before, but remembering the title doesn't mean I was reading it so just having the title ring a bell doesn't really help much.]
If there's no fic index and no memories for that story, then I shrug and wander off to the next thing on my Flist. If there are memories but they're out of order and missing a few and just generally annoying (as memories often are IMO) then we're back to balancing how much I want to read this story with how annoying finding each chapter will be. Especially since we're on Chapter Fifty-Four and I probably don't have time to read all of it right now, so I'd really like something I can bookmark that'll let me quickly and easily find my place again when I come back to it.
My POV, though, is that unless a story is just fantastically wonderful, how easy it is for me to read it matters a lot. If the writer can't be bothered to make it easy for me to find my place, to find Chapter One, to move from one chapter to the next, or from this chapter to the previous chapter if my memory needs refreshing, then I probably can't be bothered to read their story. If I have to go digging through the Calendar section of someone's journal -- or worse, the Calendar section of a busy fic community because the writer posted directly to the community itself -- then I'm probably not going to bother. It doesn't take a lot of effort to put in links. When I'm the writer, I link my own multi-part stories up, down and sideways but a writer can satisfy my reader persona perfectly well by linking each chapter to the immediately previous chapter, the immediate next chapter and Chapter One. If a writer can't be bothered then that says something about her or his attitude toward her or his readers. [shrug]
I'm not trying to say that every writer in fandom should cater to my personal quirks. Rather, that a writer who wants to be read by the largest possible audience should try to cover as many bases as they can think of, and then a few more just for the heck of it. Just because your best friend or your most rabid fan reads everything you post and can babble on about every story you've ever written doesn't mean that posting to a fic community with "Story Title -- Chapter Eighteen" and a link is the best way to go. I've seen more than one writer do that and unless I'm the best friend or the rabid fan, I shrug and scroll on by. And posting a sentence from this chapter's sex scene as a "summary" doesn't really help the readers remember this particular story. And I'm sorry, but if you're on Chapter Twenty-One, I'm not going to click through to a story I can't remember having read the first twenty chapters of no matter how hot that one sentence sounds. :P
I can't think of any reason why anyone would want to be able to click directly from Chapter Twelve to Chapter Seven, but just in case anyone does, both the Chapter Twelves I've posted have links directly to the relevant Chapter Seven, and all the other previous chapters, and to Chapter Thirteen. Just because I wouldn't want to doesn't mean someone else wouldn't want to and it's not like it takes any extra effort to maintain a list of all the previous chapter links as I go, right? And I'm not really impressed by tags but everything's tagged anyway. And I really hate memories but there's a memory link to my fiction index.
The point is to make your stories as accessible as possible, to the greatest variety of readers. If your multi-part story has a reasonably memorable title, an interesting and descriptive summary, and the header has one-click access to the pretty much anywhere your readers might want to go to refresh their memories, then you've done all you can to ensure that bringing a WIP back from hiatus is as painless as possible. After that it's up to the readers.