How you dress and act (body language) before you begin to speak is sending thousands of messages to your audience. One of the most important thoughts your audience is thinking when they first see you that dramatically impacts the first moments of your presentation is:
Who do you remind me of?
They are making a connection between what you look like and anyone else in their life who looks the same. If you are speaking to high school students and you are dressed in a standard suit and tie, they are probably connecting you to their principal or teachers. If you are dressed in a fancy suit and tie, they might be connecting you with a famous person. If you are dressed in a nice shirt, jeans and cool shoes, they might be seeing the person sitting in their chair when they look at you.
The effectiveness of your presentation's first moments is directly influenced by who you remind them of before you start talking. The question you need to ask is, "Is what they are thinking a good thing or not?" Of course, the answer to this question is complicated and it all depends on which part of your personality and style you need to emphasize to best get your point across that day for that audience. Do you need to come across as serious, entirely credible, cool, common, extraordinary, etc.? Ultimately you want to be a little of all of these, but you are leaving a strong impression of only one or two.
If you don't know which direction to take, a good benchmark to shoot for is to dress one step above the norm of your audience's attire. If I am speaking to high school students, I am wearing jeans, nice shoes and either a sport coat or a nice polo. If I am speaking to a group of executives in business casual, I am wearing the nicest business casual I have. If I am speaking at a suit and tie event, I am shooting for being the nicest dressed suit and tie guy in the room.
Regarding my body language, my method is to mirror their body language, but add just a tough of energy to it. This "touch" could manifest itself by walking a little faster, talking a little faster, smiling a little stronger, meeting a few more people, etc.
The trick with both areas is not to go too far. Now, I do know certain speakers who have the way they dress as part of their act. They either dress way down or way up and that is their "signature" thing. Of course, if everyone started doing what they did, they would pick a new thing.
Remember, your presentation starts before you start speaking. Make those few precious moments as strategic as possible. You can never get them back.
