Problems saying no

So basically every week my boss will call me up and ask me to pick up a shift or two because someone called in sick or just didn't show up. Now I really don't mind doing it because its more money and all, but it gets a little old after about 3 months. My roommate works at the same place I do and she is constantly covering for other people and is sick of it as well. Like I don't want to leave my boss hanging because shes an awesome lady, but seriously fire these people who are not showing up for work or have them find someone to work for them. It's really not that difficult.