How should you dress if you're giving a speech? There's no one perfect thing to wear.
The main thing to think about, is you've got to dress in a way that doesn't confuse your audience. That augments your message. If you're a world renowned outdoor wildlife photographer, I wouldn't wear a conservative business suit. On the other hand, if you are a bond manager and you're dealing with people's money, and their investments, I wouldn't wear shorts and a Hawaiian shirt.
You need to dress in a way where people connect what it is you do, what your message is, and who you are. For most people most of the time, you don't want people focusing on what you're wearing. You want them focusing on your message. Now, if you're a fashion designer, obviously you want to demonstrate your clothes that you design. You want people really noticing what you're wearing. 'Oh, isn't that beautiful. I'd like to get that.' But if you're not talking about fashion, and you're talking about some aspect of your business that's any kind of a traditional business, then the focus shouldn't be on what you're wearing.
Every industry is a little bit different. If you're in the banking industry in New York, then if you don't wear a suit and tie, that can look weird. If you're in the high-tech arena out in Silicon Valley, wearing a tie will confuse people. It will look stupid. It will show you're not a part of that community.
So you've always got to figure out what is your audience wearing? What do they think of you? And what is the image they have for someone like you? That should be the focus.
Other basics. You need to realize people are looking at every inch of you. So if you need new shoes, or a polish, make sure your shoes are polished because people will notice it. If you put on a few pounds, and you can't button your jacket, you've got to go to the tailor quickly. Nothing's worse than seeing somebody speaking, and their jacket is so tight you're worried that, 'Uh-oh. That button's going to pop and put out my eye.'
Every single little detail like that can be noticed. You need to be comfortable, but you need to be dressed in a way that augments your message.