I've heard some presentations recently where the speakers were clearly unprepared. The presentations weren't necessarily bad, but I could just tell that the speakers were making it up as they went along. When they were told they had two minutes left, they seemed surprised and had to scramble to fit in the rest of the information they wanted to cover, because they had spent eight minutes rambling fairly aimlessly.
When you have a limited amount of time to speak (and who doesn't?), you want to make sure you're getting your core message and most critical and valuable information to your audience. The only way to ensure this happens is to prepare in advance!
At minimum, you should:
1. Think about what your audience is going to want to learn and find valuable and relevant.
2. Write down some notes based on this, covering three main points in a logical sequence.
3. Practice several times so you know how much time the presentation takes and so you are sure that you're covering all the points you want to make.
This is a drastically abbreviated version of what you should do to prepare for a presentation, but what I'm trying to say here is this: If you don't get your message across, you have only yourself to blame. Why waste any speaking opportunity by throwing something together at the last minute, or worse, not preparing at all?
Do you want people to understand how you can help them? Do you want people to feel intrigued enough to call you later, hire your service or buy your product? Do you want people to tell their friends about you?
If you are kicking yourself later because you forgot to say something important, something that could lead to one of the previously mentioned outcomes, is it because you were winging it? Is it because you hadn't taken the time to figure out what your important points even were?
Don't throw away any speaking engagement. You have an audience. You have their ear and their attention. Make the most of it: Prepare!