Don't Forget to Mention
Despite the basic rules, babysitting should be a fun experience, so make sure to mention where drinks and snacks can be found and that your sitter is welcome to eat the entire pantry if she feels like it. Teach her how to work the remote control, satellite box, or whatever electronic configuration you have. Make life easier on her by dialing your cell phone or the number where you can be reached right before you leave the house. Then she can just hit "redial" to contact you in a hurry.
On the topic of emergency info, make sure yours is posted clearly and that you've pointed it out to the babysitter. Always include your address and directions to your home with the names of your cross streets, since emergency personnel will ask for this information and your sitter won't have it memorized.