Nothing is better than finding a good babysitter you and the kids love and trust. Unfortunately, if you use teenage girls to babysit, they keep growing up and leaving!
When our kids were very young, we had a very responsible 11-year-old neighbor come to the house every day at 5 p.m. She stayed for an hour while I cooked dinner, fed the baby in peace, folded laundry or whatever. Since I was home, this is what is called a Mother's Helper, and it was my first lesson into the value of young helpers.
Do you use teenage girls as babysitters? If you do, what age do you let them start? Do you insist that they take the babysitting course first? Do you pay your sitter more if they drive? What about household chores; do you expect them to clean up any games or toys they were playing with? Do you expect them to make dinner? How do you feel about them giving your children a bath - especially if they are the opposite gender from your children? Good babysitters are very valuable, do you share yours with friends?
One important lesson I have learned that I will share is that if you find a good one and you want them to choose you over another babysitting job, treat them well and pay them what they're worth.