Writing something to be read aloud is much different from writing for print--just ask the average commencement speaker. There are many places where they coincide, as many great writers also read well (Douglas Adams, rest in peace, comes to mind as someone who probably wrote better dialog than actual fiction). But after watching some people at work struggle through writing for speech, I think that it's a definite stumbling block.
As with all aspects of public speaking (and really, with most skills), one of the best ways to improve the writing side is to practice it. Write a lot, and as you write, read it aloud. You'll pick up on a lot of the following points yourself as you do, and you'll start to get a feel for the kinds of phrases that look stupid on paper but sound great or vice versa.
Why is writing for speech different from print writing? Let's look at the audience and the environment. Writing on a page is static, and the audience can interact with it at leisure. They can look up words they don't understand, and they can jump back through the clauses of complex sentences until they piece together the whole meaning. There's much less interference between the reader and the symbols on the page, existing in a black-on-white, high-contrast medium.
In contrast, spoken presentations take place in realtime. They can't be paused, and the viewer can't go back if they didn't understand something. Environmental noise and distractions can detract from comprehension. On the other hand, a good presenter can be far more captivating than text, and has the advantage of a number of non-verbal and pan-verbal cues.
So far, this is pretty basic stuff. But believe it or not, this is where I see most people tripping up, because they don't take into account the difference between realtime and static. That difference informs a great deal of the art of public speaking--from structure to gestures to sentence complexity. It is all about making sure that the audience a) is not lost, and b) if lost, can easily pick up the thread again. Once we understand that theoretical shift to realtime communication, we can start putting it into concrete practice.
Start by writing shorter sentences. This is hard for a lot of people to do, because they are in love with commas and parentheticals. I love subordinate clauses myself. It's also been hard in B-SPAN work for the podcasts and video blogs, because VIPs may have absurdly long titles. By the time you finish the title, a listener might have forgotten the preceding part of the sentence. A good rule of thumb should be that an audience member can always repeat the last sentence back to you verbatim without any prompting. And don't forget to make it something that you can repeat in the first place--alliteration and elaborate metaphors may look good on paper, but they're tongue-twisters up in front of all those eyes. I promise you, the time that you spend trying to untangle ambitious phrasing after a stumble will seem like the longest moments of your life. Keep things simple for your audience's sake and for your own.
Next, don't forget to introduce with an attention-getting device--also known in most circles as a snappy introduction. This is a good tip for most writing, but it's especially important for speech to grab someone's attention. It will also flatter your audience if you can weave it throughout the rest of the piece, just with occassional references. They'll feel clever, and that will incline them to pay attention.
The other key to working with the realtime constraints of public speaking is to pay close attention to structure--and that means more than just putting together an outline when writing, because most good writers will do that anyway, but specifically you need to signpost and make it explicit to your listeners so they can find out where you are. There's nothing worse than watching a speech where the structure is badly explained or nonexistent, because it will usually come across as rambling. If nothing else, listeners want to know how soon you're going to stop talking, for a variety of reasons.
First, let's go over some basics for putting an argument together in the first place. In competitive speech, which has a 10-minute time limit, the most common way of structuring a speech was generally referred to as 3.2. That means that the overall outline had three main points, each of which had two sub-points. Forensics dorks stick to this religiously, even when they shouldn't--I still catch myself laying out my freelance work this way. The real number of points and sub-points isn't really important, although I think people have trouble following if either one gets too numerous.
For the purposes of this discussion, though, let's use 3.2 as a good starting point. I actually think there's a lot of rhetorical strength to this structure (particularly compared to the other forensics stalwart, 2.2 structure). For one thing, Western culture tends to be very receptive to the number three. We're unconsciously primed for it, from three little pigs to the old formulation of thesis-antithesis-synthesis. It tends to resonate with an audience, and it's easy to put together a number of easy-to-remember outline points for your speech:
Each of those points then gets two sub-points (the ".2" of 3.2 structure). So, for example, if I were giving a speech on dangerous electrical wiring, my first point might be to talk about the causes of faulty home wiring. My first sub-point would be how most older homes are designed for a much lower load than our modern appliances demand. My second sub-point might talk about the now-obselete fad of using aluminum for home wiring, which proves highly flammable at contact points. In doing so, I've broken down my main point into a series of smaller subjects so the audience can follow me through them. You don't have to have only two--you might have many more--but two points is a nice number for someone to maintain mentally under their map of the three main points. It's also easy to gesture to them, as I'll discuss under delivery.
But either way, once you've worked all these points out and added an introduction and conclusion, you've got yourself a structure. Now you just have to communicate it to your audience. You can do that through internal previews. Basically, you're going to take every possible opportunity to rermind the audience of where you are, what you just finished, and where you're going. It will help them follow you, as well as giving them a mental layout of your argument so that they'll remember it later.
The way I was taught to do internal previews, and I think it works well, is to place them at the end of the introduction (for the main points), at the start of each main point (for the sub-points), again at the end of each main point (to review the sub-points and preview the next main point), and before the conclusion (refresh the audience's memory of the overall structure before driving your argument home). It sounds repetitive. It looks repetitive on the page. But trust me, if you do this for your audience, they will thank you.
Let me show, rather than tell, now. Here's the intro and previews from my most successful Forensics speech (a persuasive number that really was about home wiring):
According to the National Electric Safety Foundation web site, last updated March 11, 2002, faulty or aging electrical wiring causes more than 40 thousand house fires each year, taking more than 350 lives. Even worse, faulty wiring isn't just a problem on Green Acres any more: instead, the aging of American homes is turning it into an epidemic that's wired for fire. If you live in a dorm, cheap student housing, or a low-rent neighborhood because the pay in education is just on this side of abysmal... pay attention, as we first flip the switch on the problems of aging electrical work, then curse in the dark at the causes, and finally fumble for the circuit breakers to turn on the solutions.
The problem with this wiring crisis can be summed up simply: it burns your house down. Apart from that surprise, the problem is rooted in two key areas: older houses, and improperly wired systems.
[...explanation of the problem...]
Now it's true that these firetrap electrical systems are partly caused by age, but that's not the whole story. The causes also include rising electrical demand, and widespread public ignorance.
[...explanation of the causes...]
So the solution is bound to be expensive, right? After all, electricians aren't known for their charity work or low, low prices. Luckily, keeping older houses electrically safe isn't a serious drain on your pocketbook, as long as you take proper precautions and use some common sense.
[...explanation of the solutions...]
Denny Morgenstern doesn't see a lot of exploding electrical meters-and that's just fine by him. But the fire hazard from aging electrical systems makes him nervous enough, and it's a problem that more of us should be worried about. After seeing how older homes are wired for fire, and then seeing why this problem keeps blazing up, we can cool things off with some easy-but effective-solutions. As Morgenstern states, "those fires shouldn't happen." There's no way to predict what could happen, but home fires are among the easiest disasters to prevent.
I know, it's probably more than a little cheesy. That was kind of my shtick (still is, if you listen to the B-SPAN podcasts). Still, it works, and the use of a little cute wordplay makes it less egregiously blatant for the listener. You can even use these previews in written writing, and many do (particularly in academic papers), but they're essential for realtime speech.
As I was writing this, I realized that I'm not really qualified to teach anyone to write brilliantly. I'm proud of my writing, but that takes some serious hubris. But if I'm not prepared to instruct on stylistic brilliance, I am confident of the basics of rhetoric. These two main areas (simple sentences and previewed structure) are the foundation of good speechwriting. They'll rarely dazzle--only speech dorks will come up to you later and say "wow, I really loved your preview to the second point"--but they give you the rhetorical framework you need for the fireworks. As we'll see, these basics serve two roles: they not only provide redundancy for the fleeting speed of the spoken word, but they build room into your speech for the use of effective physical delivery. That's a post for another day.