With a short speech, splitting it into three parts is simple and smart. It's natural for people -- audiences and speakers -- remember things in threes.
A keynote is different. There's a huge chasm between a 3 minute speech and a 30 minute speech, between 300 words and 3,000 words.
The longer a speech is, the easier it is to get lost in the text. Even with a good outline, you will get lost.
It might take a couple hours or half a day to write a short speech. A keynote speech can take a week, or two weeks, to draft and edit and finalize.
Organization and outlining can save you endless hours of rewrites.
1) Start at the end
What do you want the audience to do? In a perfect world, what would every person be inspired to get up and do after listening to the speech?
Everything in your speech should build up to that call to arms. If you bring the audience there in a straight line, or telegraph it, the speech will be boring.
2) Split it up into three parts -- and split those parts into three
It's tough to tackle a speech 30 minute speech by splitting it into three sections of 10 minutes apieces. Take those three sections and divide them into three.
Now you've got nine total sections of roughly three minutes apiece, and you can work on them independently until they're polished.
This isn't an iron-clad rule. It's a technique. It's a good way to make sure each part of your speech is balanced.
If you simply write out the entire text, the structure might get lost. You might spend 15 minutes describing a problem and two minutes on the solution -- and not know it.
3) Variety, variety, variety
If your speech is nothing but statistics, the audience will look for sharp objects and blunt instruments.
Any sort of repetition must have a purpose. Too much of a good thing turns bad.
A solid keynote speech has variety: stories about real people, examples from history, metaphors, interesting numbers, new ideas.
A great speech doesn't simply string those things together. It weaves them like a tapestry and takes the audience on a roller coaster.
Look at each section of the speech and ask yourself, what emotion will the audience feel? What about the section before and the section after?
It shouldn't be random or repetitive. Even if you're talking about a tough issue, like hunger or the bad economy, there's a reason you're speaking -- and that reason is hope for change. Thirty minutes of "this is a horrible, horrible problem" and "thank you for letting me speak" won't work.
You have to give the audience hope that they can make a difference. Explicitly saying "You can do something" is too direct; in Hollywood, they'd say that dialogue is "on the nose."
Tell real stories about average people -- not billionaires or geniuses -- who made a difference. Local people, if you can.
There's probably somebody in the audience who's affected by this issue or has worked as a volunteer or advocate. That person's story is rhetorical gold. Don't start with it. End with it.