If you simply can't buckle down, put ass to chair and write it out, you can do this...
Get together with a friend, someone who wants to listen to you talk about your movie, bring an mp3 recorder (or old school tape recorder), buy them a drink or pizza or whatever, hit record and tell them all about your great idea for a film. Then simply transcribe what you've said into a Word doc.
At the very least, this makes the process somewhat social and enjoyable. But really, in the end, there will be no getting around digging in and doing it all by yourself at some point.
Or just start typing a bunch of gibberish until your brain gets warmed up.
I also like using a timer which can be found at the local dollar store. Set it for 5 minutes. Hit start then get to writing. Write ANYTHING. You'll be surprised when the beeper goes off. Then hit it again, and again, and again. Eventually you'll be setting the timer for 10, 15, maybe even 30 minutes.
Oh and when you hit start, you need to turn it away or put it behind you. You can't be staring at the timer ticking down.
Just some ideas. They work for me, but may not work for everyone. Cheers.