Every woman is different, what works for one won't work for another. You know her the best.
You need to decide whether to be straight-forward and tell her your desires outright (is she a kinky one), or whether to take it slow and introduce her into it slowly. By slowly I mean for example, watching a movie together that has a facesitting sex scene, and mentioning how hot you find it.
If she's open-minded and you just gave her an idea of what you might like, the next time you have sex she might want to try to please you by planting herself on top of your head if you're lucky! If she's just straddling your torso during a foreplay, try gently pushing her up towards your face. You can then start letting know how much it turns you on and how exactly you like it done. If you're good with your tongue and she'll get to enjoy it a lot, it could soon become both of yours' favourite position. I find it crucial you make sure you get to please her well when she facesits you, so she wants to do it regularly.
Some women have trouble letting 'emselves 'loose' in that position, they get too self-conscious. A glass of wine or a longer foreplay might help. Whatever you do, don't ever pressure or beg her to do it. Do let her know how hot&sexy it feels to have her on top of you like that, and how great she looks.
PS You might find you won't enjoy the extreme version of facesitting, when the total panic takes over. It is not a pleasant feeling, at least to me. It might scare her off too, so don't ask her to go hard-core on you too fast. My .05