Meet with as many members of your department as you can before choosing a supervisor. This will give you the chance to see whose personality clicks well with yours and whose does not.
Choose a problem or area in your department that interests you. Find a faculty member in your department who specializes in this area or a similar one, then see if the project he's working on excites you. A project that doesn't interest you can make your PhD program boring and unfulfilling.
Talk to your potential supervisor to see whether you have good personal chemistry. If you don't like her, you won't work well with her no matter how interested you are in her project.
Talk to other students in your potential supervisor's research group to see how they like working with him. Don't take any one student's word as law; a student may simply not click with the supervisor's personality or leadership style, or might disagree with the supervisor's interpretation of data.
Consider how much supervision you'll need. In general, senior academics are more knowledgeable but have less time to devote to your project, while junior academics are less experienced but have more time to help you. Of course, there are exceptions, so consider your impressions from meeting the professor and her research group.