Why are you so willing to sacrifice time with your family, having a relaxing evening, getting a good night's sleep in order to help her so much? You offered to help in a pinch--to me that is a once every month or two situation that comes up at the last minute. She is using you a lot as unpaid daycare so she doesn't have to shell out money for a sitter or get to know anyone else/go to the trouble of finding anyone else.
You have gotten several good statements to use from pp's. I actually agree with the ones who say to keep it simple. the next time she calls tell her "I can't help you tonight. I have plans. Hope you can find someone. Got to go." Do not JADE. After doing this two or three times, she will probably start in on you, so then you can say to her something along the line of "I offered to help you out in a pinch, a real emergency situation, not on a weekly basis. I have a full time job and want to spend my free time with my family. So, I won't be helping you out any more except in a real emergency, at the most once a month." She will probably get mad about this, if she does, she does. I wouldn't worry about the friendship at this point as she certainly isn't, except in what she can get out of you.
Try it. The first time will be the hardest and it will get easier each time. After you tell her no, focus on the time you are spending with your family or doing things you want to do.