From: Tippus Tailus
Use a piece of red stretch fabric. Stuff it like a tail. Sew the seam to the floor of the mouth. Additionally, the middle can be sewn to flatten the tongue. However, unless the character has a very oversized tongue, the feature can be omitted entirely. Consider the case where the view from the fursuit head is through the mouth. It is preferable to have the inside of the mouth completely black. Furthermore, if a fursuiter is looking through the mouth then the top of the head is likely to be seven foot high. Not being able to see a tongue is consistant with a person of this stature. It doesn't matter that it should be possible to see a tongue. It maintains an illusion consistant with having a fursuit head.

The most realistic tongues are from taxidermy jawsets, which you can get from any number of taxidermy supply shops. Other methods of making tongues include using felt, model magic clay, and fun foam. Fun foam tongues are good as long as you can't see below them; by folding and hot-gluing them, you can get a convincing illusion of a tongue.
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In addition: If you're using one of the above methods, you can always paint on clear nail polish to give it that 'wet' shine. Just be sure to let it dry and lose some of its fumes before you attach it to the head and wear it around, as nail polish fumes are toxic.
Dragonsfire