I'm in yr 12, but in yr 11 I didn't do many hours of revision a day and I got 9 A*s. Maybe 4 at most, but usually 2 or so. For me, the key was to do past papers--do as many as you can, I did all the ones in existence for English which is the one subject I did work really hard for--I was panicking because I got an E in one of my mocks.
For language students, just go above and beyond the criteria; I spent every Sunday afternoon for a few months going over grammar lessons on the about.com section for my language (French) and ended up doing the exams early--I did an AQA GCSE and they do not expect that much of you! If you can even begin to get your head around complicated language, the exam at GCSE will seem like a piece of cake. Speaking and writing are harder but you can memorize the speaking and get away with it, so have someone look whatever you plan to say over and you'll be fine.
And at the end of the day--don't panic. GCSEs don't determine nearly as much as other exams you'll take in the long run. Practice papers are key and don't stress--a good attitude goes a long way, especially in writing exams, to make your work sound confident and assured which appeals to examiners.
OCR 21st century science takers, the CGP revision guides are honestly completely enough. I wrote notes from that like one night before each exam and was fine, and the grade boundaries are such a blessing with that board. They'll go down.
My other advice is if you have any course-works which have been marked but not sent off yet, make sure you get a second opinion, or third. I had nasty surprises with almost all my coursework going down in moderation, loosing me an A* in one of my subjects I thought was for definite based on my teacher's marking. Of course it's a bit hard to tell your teacher 'I don't trust you' but encourage them to be really harsh or something.
Also make sure you structure revision efficiently! GCSEs tend to be spread out over a long time so if you have a week before your final exam, don't revise for it before that week. I also stopped going into school, which they let me do after getting permission from my parents, so if you desperately need extra time and don't feel you're covering a lot in class, go for that. If your school gives you revision packs, use them, they're usually enough for most GCSE papers, particularly essay-based subjects like R.E.