It looks perfect because she has a personal stylist. Stylists Job is to make her look perfect. Any given photo shoot, she will have four stylists, one for each side of her head . Take a picture of her to a salon , and they will help. She gets up with bed head like everyone else . : )
The problem with magazine images (despite any products that they might be advertising), is that it is literally impossible to get your hair exactly the same. They take one moment in time, and make sure everything looks perfect before taking the picture, and I guarantee you that within a minute or two after, it doesn't look nearly as good any more.
In general it's probably a combination of products, if you look up "Kristen Stewart hair products" there are a few different articles that come up, including the following that might be helpful...
I agree with what everyone else said. But not only that, you also have to take into consideration that when you look at her picture in and on magazines you have to remember she has been photo shopped to look flawless.