How do you control urges to "vent" about others?

I have someone I work with that I consider a friend, but not a true friend. She can be good sometimes but she has a side I do not like. She acts great to customers or bosses but with her equals she is a major two face. She’ll act like she wants to help us out but even talks about her previous co worker (my best friend) like she’s better than him. So it really irks me and I continue to talk bad about her. I’m fed up with her insincerity, but talking about her so much just makes me feel as low as her.

What do you guys do in a situation like this?