Mannequins both freak me out and fascinate me at the same time, espeially the faceless ones but when you use them as a photographic subject there are a couple of things you should keep in mind.
Firstly, you’d often be shooting them through glass windows, so there are the reflections you will need to consider.
One way to combat this is to press your lens right up against the glass, but this will limit the angles you have available.
The other is just to use those reflections to your advantage, and if you’re careful with the compositions, they can add a real sense of depth to the photo.
Also, depending on the display, the mannequins will usually be bathed in the most fantastic light, but this also means there will be some harsh shadows as well.
Again, I suggest going with it and including them in your composition.
Lastly, I have found that a lot is happening in those window displays so depending on your choice of lens, you may have to carefully pick out specific features to concentrate on otherwise your pictures may end up very busy and cluttered.
One thing that appeals to me about taking photos of these freaky things is that can serve as good practice for when you are working with posing a real person.
The people that arrange these window displays tend to take a lot of care and attention to every detail, especially in the higher end retail stores, so it seems a shame to let all of that effort go to waste by merely walking by.