It looks as though my plan to get Sarah to take more responsibility for getting good grades is working. A few months ago, after fighting with Sarah over her lack of turning in her homework on time (if at all), I decided to just give up and let her be responsible for her own fate. I couldn't take fighting with her over it one minute longer.
Of course, I couldn't just let her get away with letting her grades slide, either. So, I made a deal with her. The deal was that if she gets straight A's on her report cards for the second semester of seventh grade and the first semester of eight grade, then she will get her own laptop computer. This seemed like a great deal to me because I don't have to yell at her about her school work anymore and she has an incentive to make sure she gets good grades in school.
Today, Sarah brought home her first report card since we made the deal. She got straight A's! So, now she just needs to continue with the good grades for three more report cards and she'll get her laptop.