Write everything down. It's not always easy to remember everything you say and a note really helps. Better yet, type everything out. Your babysitter doesn't want to have to call you asking what rule number five says because she can't read it. Your babysitter hates to interrupt you when you are out. Speaking of interrupting you, your babysitter will want to know when to interrupt you. If your kids miss you, should she call you so they can hear your voice? Or should your babysitter only call you if there is an emergency? Should your babysitter text you about questions? Be clear on how and when your babysitter should be contacting you.
Negotiate the hourly fee upfront. Your babysitter is here to help, but she is also there to get paid. Get that out of the way before you even hire your babysitter so there are no last minute issues. At the same time, let your babysitter know what you expect of her in terms of services. Is your babysitter supposed to wash the dishes while you are gone? At the same time, know that she is your babysitter, not your maid.
Say goodbye to your kids before you go. Yes, your children may be sad, but it teaches them to trust you. They won't go looking for you aimlessly and it leaves your babysitter with a less fussy child too! When you leave to avoid the crying, all you're doing is forcing your babysitter to hear it amplified longer ... unnecessarily.
Don't make your babysitter drive your kids in his/her car. You know this firsthand: kids are messy. If your babysitter is going to drive your kids around, she would prefer to use your family vehicle since it has the car seats properly installed anyway. If you absolutely must make him/her drive your kids in her car, compensate the babysitter for mileage. It's only fair.
Don't micromanage. If you're texting every 10 minutes, it makes it hard for your babysitter to focus on your kids. Your babysitter loves when you check in, but not when you're bombarding him/her with calls and texts. Be respectful to your babysitter and your kids will usually mimic your actions.
Give feedback. When your kids tell you they loved the babysitter, share that with her! Your babysitter takes a lot of pride in what she does and that is the best "thank you" she can ask for!