Story, story, story; the framework upon which any tale is constructed.
Without a story, there is nothing; no meaningful interraction, no conflict, no nothing. It's impossible to create a movie without a story (ok, well, maybe not impossible, but certainly very difficult); even a one scene film has SOMETHING which it is telling the viewer, even if it's not much of any interest!
When writing, always have at least an IDEA of where you want to end up; without an end goal, you will wander about all over the place and, although you might eventually get to where you want to be, it may very well have taken a lot of wandering about to get there :)
When writing for Odyssey, I usually start with a 3-4 line synopsis listing the key points of the story. Each episode script is aimed at around 38-43 pages, which should translate to 35-40 mins of on-screen time when filmed and edited. I'm a firm believer in writing character first and foremost (more on this later), developing the persona of the actor in the role through the telling of the story, and showing how the characters handle the situation they find themselves in.
It might not always be evident at first sight what the story is actually about, but each episode so far written has shown a little more of the principal characters, building on their burgeoning personalities, back stories and interractions with others as they face the issues in that episode. Hopefully, this rounds them out as more "real" and builds believeable characters which will grow and develop as the series progresses.
With one-offs or mini series, its a lot harder to develop character, so in these cases the plot itself is of at least equal importance. These tend to be much more rigid in their structure, at least for me; I'll do a broad-strokes run through of at least the first half of the movie in very rough form, to give me an idea of the key conflict moments, set piece scenes, snippets of dialogue, etc. I then use this as a "guide" when writing the full script.
That's not to say that I stick rigidly to this "route planner", but it allows me to avoid the "wandering pen" trap that a lot of writers can fall into; ambling aimlessly around from one situation to another until the bemused characters reach the end :)
I find that using the plan allows me to keep it tight, focus on the emotions and scenes I want to portray and stay on track, rather than going down the "scenic route" :) It also allows me to insert specific moments into the script and plan towards them, but again this is a flexible thing that grows out of the writing process as the characters develop on the page.
My scripts tend to be light on the camera and shooting notes, at least when I write for myself. A brief note of the specific shot set-up is all I tend to use, as I usually work "on the fly" when shooting, as a perfectly sound idea on paper doesn't always translate as well onto film. I also tend to write "instinctively"; I never worry about particular set-ups until I come to film it. If I've written "character shakes the phone", I will work around it (as there is no suitable scene in TM to do this). I don't try to constrain my imagination, as I find that blocks up the creative channels; write the script how it appears in your mind, then try to find the right scene to shoot it (whether that be direct out of the game, a combo scene utilising creative camera angles and set dressings to maintain the mood or even planning up chromakeyed shots to insert a particular scene in where you want it).