APPAREL
Suits
For business and formal speech occasions, suits (for both men and women) are entirely proper. Additionally, ladies may be able to “get away” with an ensemble that wears like a suit but isn’t “officially” one (such as a matching blouse and skirt combination.)
If you already have a suit, make sure it’s clean, pressed, and void of lint. If you don’t own a suit, consider buying one (or borrowing one) if your speech is extremely important.
“Dressy” Casual Attire
Some presentation environments lend themselves to more casual (but still conservative) attire. When in doubt on how to dress, ask your host (or the person who “booked” you if you’re a professional, paid public speaker) for clothing advice.
Though it’s generally better to be a little overdressed, you don’t want to seem out-of-place, as it will make both you and your audience members feel uncomfortable.