How do you handle a woman's insecurities?

I have run into this problem pretty frequently. Sometimes she'll be jealous of a woman in my life, othertimes she'll feel like she isn't enough for the job she wants. Hell, I've run into girls who get jealous of their sister-in-law or are struggling to get mommies approval. I get how they feel, but I honestly choke every time.

How do you soothe a situation over like that?