How can we advertise our dog walking services?

My friend and I are trying to earn some money by offering dog walking services for cheap in our neighborhood. Problem is we are having difficulties advertising it. We tried posting posters on public bulletin boards but they all got taken down because it needs to be non-profit advertisements on the board. We are going to try hiring one of my more popular friends to advertise to her friends at school, but I wanted to know how else we could advertise. Any suggestions?