How do I comfort a 3 year old when she doesn't want her mom to leave.?

I'm going to start babysitting this 3 year old girl, sometimes in the evenings for my mom's friend. I've met the little girl she's shy but sweet. Her mom told me that whenever she has to leave her daughter with a babysitter, that she cries hysterically. How do I comfort, and help calm down a little girl who doesn't want her mom to leave? I'll be babysitting her in the eveings, and putting her to bed, she won't see her mom until the next day. Any ideas?