Choose the doula that you fall in love with. Make sure you like the way she smells [because senses are heightened during labor]. Make sure that when she looks in your eyes during the interview, you're going to want that face next to yours during birth. The spiritual definition of love is that when you look at the person it makes you love yourself more, and it's instantaneous.
Then, make sure her references are good. Does she have any colleagues or clients she could offer as references? Is she certified through an accredited organization, such as DONA? Sometimes couples are not able to afford a professional, certified doula, but there are so many doulas-in-training who have plenty of training, plenty of heart, but not the official "certificate" yet. A doula-in-training with good references is as fine a choice as a certified doula.
Ask yourself: Does she answer all of your questions? Would you like to get to know her well? Imagine you're having a really long labor, and you're hearing this woman talk to you for 33 hours. How does that sit with you?
Spend time talking to different doulas. The most important thing is the connection that you feel with the doula you choose. Experience is important, but the rapport makes the biggest difference.