Frustrated!!

I am so frustrated with Erin these days. She is terrific kid BUT I am having trouble motivating her to do anything more than hang out with friends. Her grades are OK but not as great as they could be and now that she is not on a soccer team she is not exercising and getting a bit lazy & making poor choices with food. What can I do?
With regards to school, we have tried everything. We have yelled, offered incentives for good grades and taken away privileges bad grades. Nothing seems to work. We let her know that she cannot get her drivers license unless she has a "B" average. At the end of the school year she didn't have any "D's" or "F's" (which is great) but she didn't have the B average. This summer she is taking Spanish III at College of the Canyons (to get ahead) and we told her that if she got an "A" in the class she could get her license. Well yesterday she told me that if she does well on her final she is certain to get a "B" in the class. She was not upset with this. In fact, she seemed just fine with it. I know I don't really have a lot to complain about BUT it is hard to see her not working up to her potential. She does NOT spend a lot of time studying. She had nothing else going on this summer and could have spent the extra time to insure the "A" but she didn't. I don't know what else to do. She certainly isn't in trouble as she is passing all her classes BUT she is not putting in much effort and really not trying as hard as she can. What can further can I do to motivate her? We do follow through on our "punishments" so she knows that we mean business. How do I get this to be more important to her?
And then there is the whole sitting around issue. I want her to take care of herself physically. Discussing it with her, or any teenage girl for that matter, is a delicate issue. You never want to make weight an issue. You always want to make health an issue. I want her to want to take care of herself now and make it a life long habit. Exercise is a daily part of my routine and she knows it. I want her to do the same. Next week we will go check out a local gym which she claims she will go but we all know how that often turns out. A friend of hers already belongs and they say they have a plan to go together every other day. We will give it a try--I certainly won't lock into any long term contract. Also, school starts in 2 weeks and she will have PE. She will continue to be on the soccer team. They have a new coach who, I believe, will work them much harder. I think it will be a tough couple weeks for Erin but she should get back into the swing of things in no time after all she is still so young!!!
I guess I should consider myself very lucky that these are the issues I am dealing with. I know it could be much, much worse. I love and adore my daughter and I only want the best of everything for her. I can't do it all for her and I just want her to have the motivation to do it for herself. I want her to be the best she can be and it frustrates me to see her being lazy. I am always the optimist and I will continue to be. At some point it is going to "click"--hopefully it will be sooner, rather than later.