During the planning phase of a project I like to be - or at least feel - organised. On my first projects I began by having lots of stuff written down which I would lug around, but not often reference. Sometimes while at work shooting something I'd realise the one thing I really needed was on my home computer and inaccessible from my current location.
Thank the cosmic valkyries for Google Documents. You work on your files from any computer via your google account and they will be saved among your google documents for access from any computer. This is great news for me.
I use a lot of documents during prep. Mark has made frequent reference to the barrage of documents and spreadsheets I bring out during rehearsals or when script editing. I have spreadsheets that I reference frequently, such as the invaluable Locations spreadsheet, which lists out all the scenes and locations for the filming. It also tells me who appears in which scenes and also allows me to see which scenes have the most actors and which actors have the most scenes (really useful when planning a shoot).
Then I have spreadsheets I don't reference much beyond making them, such as the character relationship sheet, which has all the character names going up the side and along the top, and I write about character A's relationship with character B where they intersect. Interestingly, but crucially, A's relationship with B is often different from B's relationship to A. It's rarely the same relationship from both perspectives. Although I rarely go back to it, the act of writing it is invaluable and revealing, and also lets you know pretty quickly which characters you don't have a grasp on, and where you need to do some more exploring.
I also have a spreadsheet with all my extras and their contact details, a document outlining issues that crop up when planning certain scenes, which is a really great place to put all my ideas and solutions for working around these problems. I have one containing ideas for the current project and another for music video ideas (best to write them down before they disappear into the ether). I have some "drawings" on google documents as well, looking like a big flow chart which allow me to visualise the scene flow and the throughlines for each character as well as their objectives in each scene.
Sure, sometimes the google versions of spreadsheets and word documents aren't exactly perfect or fully featured, but it's a small price to pay for the convenience of accessing them anywhere any time, without having to spend time copying everything over to a memory stick every time you change something. I know a lot of people find word processing cold, and prefer to work with pen and paper, which is (of course) preferable when working with the creative side of things (I do this too, I always keep a notebook handy, doodling seems to unlock the inner artist), but for organisation and planning, sometimes you can't beat a good spreadsheet, much maligned as they are! It also means I can spend downtime at work tinkering about with all the difference facets of the production, which keeps me immersed in the project and keeps my thinking up to date.
So, spreadsheets! Pretty boring subject matter, I know! But sometimes it really pays to have this stuff available for quick reference, and it also means I feel a hell of a lot less stressed about the project. You start finding problems that you didn't even realise existed yet, and you can apply a little creativity and solve them before you even get to shooting, which, speaking personally, is liberating.
Be as prepared as you can possibly be, but make sure you're still flexible and open on the day. You will still get hit with a lot of unexpected issues, but you'll be in a much better position to deal with them if you're not fretting about everything else.