Preparation is everything when it comes to a shoot - it can mean the difference between your cast twiddling their thumbs for five days or having your shoot DONE in five days.
When you're directing on set, you are a manager. You are managing time, people, and resources, and all manner of things can and do go wrong on a production set: Actors get sick and you have to completely rearrange the shooting schedule, equipment breaks and you can't replace it, etc. etc. But a lot of these things can be avoided with proper preparation. I myself am a big proponent of shot lists and storyboards, but you need to be adaptable. When it starts raining outside, you need to be able to pick things up and switch to an indoor shoot that wasn't scheduled until the next day. Every aspect of your shoot needs to be treated like a piece of a jigsaw puzzle that can be modified to fit into any spot. I know; easier said than done.
That being said, a filmmaker's worst enemy (especially a brand new filmmaker) is becoming obsessed with your magnum opus. As a writer, I get it; this story is your baby. If you're also directing it, you want it to be perfect. Most of your cast does not share that notion. Most of them are going to be there for different reasons. While it's exciting to have all these grand ideas racing through your head, the rest of your cast only cares about what those ideas turn into. So you need to be in tune with what your cast wants and what they can tolerate, because making a film is all about knowing when to compromise and when to stand your ground. You have to be able to exercise your will while also keeping other people's morale in mind.
My best piece of advice I can give you is to know your set like the back of your hand, and to just keep the wheels turning. Indecision is a filmmaker's second worst enemy; even if the production is devolving to something you never envisioned, just keep shooting. Your cast and crew will be happier to be in motion than to be sitting back doing nothing while you try to get everything just right.
