You should want to do well in your GCSEs for you not to prove other people wrong! It's your life!
About the past papers, I think the most effective way to use them would be to actually do them in exam conditions, then go through the mark scheme, and learn anything you don't understand. I don't necessarily do that, but I think that would be the most beneficial!
As above: do it for yourself, not for other people!
I personally wasn't too strict with myself with my GCSEs: I just made a few notes, and mostly spent my time reading and re-reading my revision books/class notes.
Get your hands on some good revision guides if you can: they should specify somewhere on the cover/towards the front of the book whether or not they cover your exam board (you know - AQA, OCR, WJEC and so on). I used the CGP revision books, if I remember rightly. But they weren't particularly cheap, so I think I only got them for the subjects I found hardest.
If there are things you don't understand, read it again/make notes until you do understand. A good way to check you've learned the stuff is to give your revision book/class notes to someone else, and either try to explain the topic to them (without looking at your notes), or get them to ask you questions.
As for past papers - it's best to do them under test conditions, I find. I never bothered doing any for GCSE, but I did a few whilst revising for my A-levels. I treat it as an exam, and mark it (using the proper mark scheme) strictly. Then I see how I've done and, if needs be, do it again after revising more (so using pencil is best, as you can rub out your old answers!)
Oh - one other thing, if you have any coursework for any of your subjects: try hard with it. GCSE coursework often accounts for quite a high percentage of the total score, and because you can get coursework marked and do it again, it's an easy way to get some marks on the board.