I use my kids as an excuse for why I need kids picked up by a certain time. I just say we need to be out of the house by a certain time to be at hockey, Brownies, dance, etc. It has worked so far, though all my parents are very respectful in that way. Not sure if you have older kids or not.
I also take care of my best friends daughter and while it is kind of awkward with money and when I need to discuss behavior issues, I love how close I am to her daughter, and that my own girls have a really close relationship with her as well.