I agree timing is everything, you need to know you have a clear 24 hours to get cracking in.
So pick a day the wife is away on a buisness trip, or that the kids are with the grandparents.
As part of that I'd suggest you tell people what you are doing, the more the merrier.
Not so they know how clever you are but so they know that is not the day to come and visit or try to phone you.
Thinking time is important and gives you a break from hardcore writing.
When you come up against something you can't figure, step away from the machine and take a break to think about it. Even just going for a walk is good. These natural breaks are good times to eat as well. That way you take a break but are still working.
Pick a small game to write.
You are not going to create a magnum opus in 24 hours. So don't try to write a 300 page corebook. A vast campaign world is going to take time you do not have. So pick a small idea that requires less system and background, such as npc stats, rules for animals, poison and falling etc. A system that uses a broad brush will also help you speed through. Less crunch (oh how I loathe that expression) and more story.
You don't need to start at page 1.
If you get stuck on something, start writing another part. You don't need to finish the introduction before you get onto character creation. If the rules system is coming together first, write that. Cut and paste is your freind
