You can read a full article and list of 6 tips here: http://www.forbes.com/sites/nickmorgan/2013/12/05/how-to-avoid-disaster-six-rules-for-what-to-wear-when-giving-a-speech/
There are no real hard and fast rules for dressing and you will probably dress differently for each speech. You don’t need to “dress up” but you do need to “dress strategically.” That means:
- Wear things that fit, are comfortable, and that feel good. If it’s too tight, too short or ill-fitting you are going to fuss with it and not feel as confident. Fidgeting with clothes (especially pulling things down or out because they are too tight) are common speech distractions that affect delivery.
- Dress slightly better than the audience. You don’t need to wear a ballgown but you want to look like you give a crap and know what you’re doing.
- Dress for your character and theme. If you are discussing football coaching then a wrap dress doesn’t really seem necessary. If you’re discussing the life of a fashion mogul, put on some accessories.
- Don’t wear distracting things that will take away from or overshadow the speech. These can be t-shirts with strong or offensive slogans that will make the audience look at the shirt and not at you, things that are loud or clunky like a bunch of bracelets (I know, the irony), or hats that obscure your face.
- If you have longer hair, put it up. If you have long hair and you let it hang you WILL play with it. It’s inevitable. So pull it back somehow, even if you just comb it back with some hair spray so it stays out of your face.