The best thing is to apply a non-greasy product with a matte finish such as 303 Aerospace Protectant. After you have washed and dried your mats, apply using very little product. Once the mat has even coverage, use a microfibre cloth (cotton is also acceptable for interior surfaces, provided they do not scratch) and scrub the entire surface of the mat until there is no product left.
When you are done, you should have a beautiful new matte finish on your mats.