Check your clothes before you leave. What do you think when someone's fly is undone? It might seem like just an embarrassing situation, but unfortunately (fortunately?) first impressions play a large part in how the audience receives a speech. Take a minute in front of the mirror to make sure you're giving the impression you want to give. Don't obsess, but do care.
More Clothing Tips:
Don't distract. Be refined, be catchy, fit the venue, but you are the focus, not your clothing (unless it's also your visual aid).
Test gestures in the mirror. Stand tall, throw your arms up, bend over, everything you might do on-stage. Make sure other people aren't seeing something you aren't!
Be too formal overdress rather than underdress.
Don't be shiny. Watch out for jewlery that reflects light. You don't people blinded when they look at you.
Wear solid colors. Ties, skirts, jackets... they convey confidence and are hard to mess up.
Check buttons and zippers. Avoid embarrassments before they happen.
Have confidence! After you're out, don't worry about how you look. It's about you and your speech, not your clothes. Clothes don't make a speech nor will they break it!