The problem with Halloween is that it is usually freezing. It is hard to properly enjoy dressing up in costume if you have to suffer being cold. I have always thought that anything that helps in that respect without detracting from the costume is a welcome thing.
The AMB mesh tops are great for this purpose. Although they are not very thick, the extra layer does help take the edge off. They also come in a variety of colors, including flesh tone, so they can be as obvious or inobvious as you like.
You can check them out at our sister site Cobweb Clothing, or see them in person at Centerstage.