When I finished the rough draft of my current feature I posted a picture on Facebook and Twitter of my index cards all crossed off. Some of you asked what they were and what I used them for, and even though I've talked about this subject before, I really think it's worth mentioning again. It basically works like this.
Go buy a pack of index card, doesn't really matter what colour, unless of course you want to use a different colour for a different act. Count out 40 of them and start writing short paragraphs of each scene you would like in your script, one to each card. You probably won't have 40 scenes in your head yet so plenty of those cards will be blank. Not to worry you're about to fill those in next.
The index cards should be split like this: 10 for the 1st act, 20 for the 2nd and 10 for the 3rd. Card 5 should be the inciting incident, card 10 the end of the first act, card 20 the midpoint, card 30 the end of act 2 and card 40 the end of your screenplay.
Now you fill the rest of those cards in and pin them up on a wall somewhere, a nice big wall so you can spread the cards out. Don't worry if you change some of your ideas as you go along, that's why you bought lots of cards, because your plot will change as you discover gaping holes in it and areas that need more work. It really helps to visualise your plot and where its faults lie. When the 40 are full, get writing that screenplay.
Cross off each card as you go and if you get a little stuck move on to another card and come back later to the one that was giving you trouble. When they're all crossed off that's your first draft complete.
Of course these are not hard and fast rules of how you should plot, but this is how I do it. Things change fast as I write and I often find scenes will change, new scenes will be added and some will be removed altogether. The index cards are only a guideline to help me focus on my characters. Of course you could go a little over board and write each characters appearance on those cards in a different colour, so it all looks very pretty, but that's up to you.