I usually have something in mind for the first chapter so I write and post that and ask for pairings if anyone wants to see anything in particular. Generally I try to sit down and push through one chapter at a time then get it posted immediately. I have one fic Ive been working on where I got half a chapter written then I kind of got stalled writing. Still it's half written and unposted (sorry FF6 people, it'll be up sometime). Similarly one of my stories I posted the scene I wanted, and didn't have any ideas where else I wanted to take it. Without feedback or new ideas I just kind of back burnered the story. It's officially ended now.
I'm working on a fic right now where I started based on one idea (which I didn't even use) and by the time I got through three chapters I had a bunch of requests. Without pairing requests I just kind of let the characters go and see what happens, which is what the third chapter is. I've always been kind of bad at dividing chapters up, even in my serious writing I separated all of my novels by days instead of by chapters. That idea works well for novel length writing with thirty something page chapters but not so well for an 8-12 page story. That's kind of why I had the first few chapters popup in rapid session around Thanksgiving. Two chapters got split into three to keep all of the similar content together.