Preparation is absolutely essential to doing good fieldwork. Some of the preparation includes obvious tasks like figuring out where you are going to go, why you are going there, and how you are going to spend your time when you get there. Figuring out how you are getting there is usually a good idea, too, as is making sure that you have any and all paperwork that you will need. That's often a bit harder for scientists, becuase there are frequently extra permits and permissions to be obtained. There's also the matter of setting up your schedule once you get on site. Time is precious, and it's common to have to budget time tightly to make sure everything gets done.
Let's put it this way: fieldwork doesn't often happen on a moment's notice. When it does, it's usually because something extrordinary has happened somewhere in the world, creating a research opportunity that might not be available for long. (Most people refer to these as 'natural disasters'.)
It's also important to make sure that the logistics for the trip are lined up ahead of time. Food and shelter come under this category, of course, but I'm mostly talking about gear. You need stuff to do science, and there's nothing worse than finding out that you aren't going to be able to get all the data you need because you forgot to bring something important. That's why you'll find scientists (like me) who would ordinarily pack for a two-week vacation without using a list putting together detailed packing checklists for fieldwork. Putting together the checklist is usually followed by staring at it for a while to make sure you haven't forgotten anything, packing it all, unpacking it to make sure you haven't forgotten something, re-packing it, and wondering what you've forgotten.
Anyone want to take bets on what I'll forget?