Mannequins exist to sell products. They are there to grab customer attention and invite them in for a closer inspection. As with anything in retail, attention to detail is crucial in grabbing the RIGHT attention.
DON'T: Where's the bottom of the front mannequin? This surely attracted attention, but the wrong kind. I noticed customers noticing it not for the clothing displayed, but rather for the lack of it. Imagine little boys laughing so hard.