I used to give a lecture, when I was teaching a short while back, called "How to write a screenplay when you can't write"
Here are the editied highlights:
1) Don't sit in front of a blank page and try to write the whole think from scratch.
2) Switch off your computer and buy a cheap notebook (pocket sized), a writing pad, some file cards and a few pens.
3) Put the small notebook and a pen in your pocket and take in everywhere (including bed)
4) Give yourself two weeks of just going about your everyday life and everytime you have an idea (good or bad) for a story, or a scene in a film just make a few notes, so you'll remember. if you see a good location that you think you might use, write that down as well.
5) At the end of the two weeks take your notes and transfer the ideas you like the best to the big pad.
6) Get your mates round and throw the ideas you've got and see which ones they run with (take notes of all the ideas, good or bad)
By this stage you'll have a rough story idea and maybe the rough outline of a few key scenes.
7) Get your file cards and write in rough the idea of the scenes you have, one card for each of the scenes. (They don't have to be connected, they are just scenes.

look at one of your rough scenes and ask yourself, if this is happening now (ie- The man searches the train for the missing suitcase) what happened before that or after that (ie- The man follows the girl to the train station), which then becomes another scene. Just repeat this process until your story/plot is complete.
9) Get some acting friends together in a large hall and explain your basic story. Then take one of the cards, explain the actions in the scene and about the characters involved and then get the actor to improvise the scene. (Tape this)
10) Talk to the actors about the bits you like and the bits that didn't work, ask them to do the same and then run the scene again. (Repeat this process until the scene rocks)
11) Do this for all your other scenes (It'll get easier as the process develops and the actors get to know the characters)
12) Turn on the computer, open a blank document in your script formating software, type what you see and the dialogue from the scenes you've already taped and the job is done.
Bingo a fully written and rehearsed script ready for you to shoot.
(works on shorts and features)