When I first started out writing articles I was really really pleased with what I had written, sent it off to the editor and was told "it's great, but you need to dumb it down". It took me a while to understand why, but eventually I realised that the reason for this was not because I was writing badly, nor was it because my audience were unable to understand me, it was because when we are looking for information we want something that is quick to read, easy to grasp and not so taxing on our brains that we just can't be bothered to read it at all.
My first tip with this is to write your speech exactly how you intend to speak it. You don't fill your normal everyday conversations with words that are long, tiring and leave people pondering their meanings, you speak in shorter, understandable sentences that are straight to the point and clear in what you want to say. This is a tough balance to achieve with speech writing; there is a fine line between making it too difficult to comprehend and sounding condescending and making it so basic that you insult your audience and cause them to assume that you are doubting their intellect. It is important to find a middle ground. As a rule of thumb, it is always better to KISS – keep it simple, stupid.
Secondly, try not to include too many details. People will switch off if you are rambling on and on about each tiny detail of your topic, regardless of whether it is a school speech or a hard sell for a multinational. Instead, put your point across using short, brief yet informative comments and offer further information in the form of leaflets or invite questions afterwards. Leave them wanting more, and if they are even remotely interested then they WILL ask for more.
Thirdly, never, ever try to cram figures or statistics into your speech. People absolutely do not retain this type of information, they will switch off almost instantaneously, and will probably find it more interesting to count the woodchips on the wall next to them. Instead, use your information to create a colourful, eye-catching chart or graph which can work alongside your spoken information. At least if the audience does somehow manage to switch off from your speech they can be looking at your interesting graphics and absorbing some of your information.