I tend to plan similar to Mare in that I have a beginning and an end in mind and maybe a few plot points that happen in the middle, but the path it takes me to get to those points changes as I write. I usually only plan a few chapters ahead as I'm writing, sometimes not even that, but I always have that end goal in mind.
That said, there are times when I have my heart set on writing one of those plot points, whether in the middle or at the end, and when I get there, I'm not sure it even still works with the story I've written up to that point because of how things have changed. I have learned the hard way what happens when you force an idea that no longer fits the story, so I'm trying to be more flexible. It's hard to abandon an idea that you've had in your head for a long time though.