I, too, run most everything on the fly. I figure up the beginning: how the group is contacted, how the "Johnson" works, and my idea of how it should probably go. Then I've got a main idea of what needs to be done. I then try to get a few ideas on how I think my group would run the mission, that gets completely blown up, and they do a bunch of stuff I don't expect. Yep, so I've pretty much stopped planning. I just try to keep up with the group.
Hell, half the time they end up doing more than the mission. As an example: they had about five hours between finding out when their target's next meet was and when it was to take place. So two of them decided that they'd like a nice fat gun in the back of the box truck of theirs, and since this was the first run, didn't have the money. So they went to their "boss" (one of the players, who the group decided had started the team) and told him they need some funds for a little side project to help "further the group." The "boss" approved it, and as a group they completely shook down a doctor's office and sold the stuff they ripped off from it.
That was completely out the blue and unscripted, as I had no clue this was going to happen. I prefer to play my games like that though. I like the players to feel like they have control. I mean, I've got control of the world, and if they screw up, they know I'll get 'em, but if they feel comfortable doing something, I dont stop them. Shoot, I even let 'em do things on each other if they want. Such as the "boss" using magic on the big Troll who doesnt respect him and never uses his real name, so that the big dumb troll calls him by name. Although I've had things like that go bad before...hey, I wouldnt be happy if I figured out you've been using magic on me either.