For women, a simple, knee-length dress with a fitted bodice and flared skirt is a good option. The fitted bodice will help support and enhance your figure, while the flared skirt will allow you to move freely and gracefully. A wrap or cardigan over the dress can add warmth and modesty, if needed.
For men, a pair of dress pants with a button-down shirt and a tie is a classic and elegant choice. A vest or blazer can add a touch of sophistication.
Whatever style you choose, make sure your slow dance costume is comfortable, flattering, and allows you to move freely. You want to feel confident and at ease when you're dancing, so you can focus on enjoying yourself.