The following is a souped-up version of a comment I made on Story Games which a lot of folks have said a lot of nice things about. I'm reproducing it here (and embellishing it a bit) in the hopes that other folks can get some use out of it.
The following will sound like 9th grade English advice on constructing an essay; it's only marginally related. This is planning and constructing a book, which is way the hell more complicated. This isn't about designing a roleplaying game. This assumes that you've already designed the game. This is about writing the game, taking the design and presenting it in a way that's comprehensible to people who are not inside your head like you are.
You probably have your setting and rules written down somewhere. That is not your manuscript. Those are notes. Now that you have completed notes, you need to write the manuscript. Before you write the manuscript, you need to plan the manuscript.
(Doing the following on index cards is really helpful; you can also do it on a word processor, or even in a plain old notebook -- but you'll use a lot, a lot of pages and get hand cramps.)
Write down a list of all the things that you'd need to explain to your intended audience*. This is not a list of rules, although the different rules will probably be items on the list. Then start shifting around the items on the list so they're in the order that you'd present them in. After you think you're done, go down the list and for each item think about what other things need to be explained before that item. Make sure they're already on the list, above that item. When you're done, you've got a procedure to teach the game.
Now chunk the list into sections. Some of the items will look like they sort of 'belong' to a previous item: make the belonger a subitem of the belongee. Some of the items will look like they sort of belong together as a group. Create a new item for the group, give it a temporary name, and make all the others subitems of the group name. You'll need to go through the list a few times. You'll get groups of groups, and subitems of subitems, and at some point, they'll all clump up into a reasonable number of supergroups. Eventually, these groupings will become chapters, but don't think of them as such yet, because you'll end up making assumptions. You have a rough sketch of what the book will look like.
Now go through the list and use indents to indicate main topics and supporting topics. Any good word processor can do this. If you can use 'Harvard' outlining, with roman numerals and then a/b/c and then 1/2/3, you'll be golden. Eventually, those become part/chapter/section/segment distinctions (with roman numerals being chapters, lowercase alpha being sections, arabic numbers being segments, and so on). Right now, just sort of get them organized. Now you have an outline.
Extra bonus: go through the whole list and write down, in one short paragraph, what gets explained in that section. This may very well make you realize that you need to add an item or two somewhere.
Payoff: You have a nice, fleshed out outline. Now you can write any section, in any order, whichever section appeals to you when you sit down at your desk. Tick them off as you go. Write all the sections separately and in different word processing files. You'll thread them together later. Do that only after you've written the vast majority of the sections (at least 90%). Then read it through yourself; you'll notice a couple bits that you left out. Write those as sections and insert them somewhere appropriate in the outline and the compiled manuscript. Then you will have a first draft, which you can hand to somebody else who can read it and tell you which bits don't make sense.
Super Awesome Payoff: Using a word processor or layout program that actually uses styles, you can define styles for different levels of headers. There are chapter headers, section headers, segment headers, and so on. How do you know what level header you should use? Refer to your outline and what level of indent that bit is at. You should be able to do this uniformly down the whole outline. Not only do you have regular styling, you also have a table of contents!
For an example of this in action, see John Wick's Livejournal, where he is posting the sections of Houses of the Blooded as he writes them. Each of those sections, and what is covered in each section, is coming off of an outline like the one I described above.
This is a lot of work, but the trick is to plan it and chunk it out so that it never feels like a lot of work. Whenever you sit down, you should be sitting down to write just one section -- a page or three. You will be astounded at how quickly you tick off the items on your outline.
Then it's time for somebody else to look at it and tell you where it doesn't make sense, and then you can start playtesting!
* You have an intented audience, even if you haven't identified it yet. What kind of person do you want to get your game to? Who do you want to play it? That's your intended audience. Write for them.