Or failing all that, I think Hully's original suggestion is probably the best! 
cyb - DS2 is just like your DD. He has just finished his GCSEs and although he passed them all (hooray!) he didn't get anything like the grades he was capable of. What really saddens, angers and frustrates me is that he seems happy to accept lower achievement.
However, I think a bit of a wake-up call for him was the realisation that - had he wanted to go to his own school's tfh form, he would have failed to get in on the grades he achieved. (Thankfullly we had already chosen an excellent 6th form college for him, where he had a confirmed place)
I've tried nagging and shouting, I've tried leaving him well alone, and neither approach seems to make any difference. Sadly, I think they do have to 'get it' for themselves.
DS1 was exactly the same. He did really poorly in his AS levels too. However, something clicked with him in his final year, and he turned CCCE into AABA*.
When I asked him recently what he thought had happened, he said he suddenly realised that he was at college with lots of other students who had come from private schools, and he didn't want them to do better than him.
I had hoped that DS2 might have learnt something from DS1 - but evidently not. Our experiences with DS1, though, mean that I am going to be much more on DS2's case about work - and making sure he is doing regular reading/homework.
I think the suggestion that was made re going into school and liaising/working with the teachers is a really good one. Personally I wish I had done this, and also wish I had asked them to give DS2 extra work. He, too, is very bright and I suspect often bored.
The big problem that our children now face is intense competition. It's all very well for those of us here to say they failed every exam but they are now Prime Minister or some such. But in this new world, many universities make their offers on the basis not only of A level results but also GCSE results. So it is imperative that if we want them to go the good/best universities that they do as well as possible in their GCSEs.