I'm a little bit rusty on the content of my GCSEs (currently a mathsy Year 13), but here's my two pennies' worth...
Maths-wise, the usual Bitesize/My Maths sort of stuff is pretty comprehensive. Make sure to do plenty of past papers - if you want, try doing a few papers from different exam boards to look at different ways of phrasing the same question, but it's not totally necessary. The Further Mathematics Support Program has some GCSE revision videos here - I can't comment on the quality of these particular videos, but their A-level ones are excellent.
For Literature, go to SparkNotes and find the pages for your texts. Memorise a few points to combine with what you've learnt in class. From what I remember, my Lit GCSE was mostly phrasing my blagging in such a way that it sounded profound. I barely remember Language at all, other than I always struggled with timekeeping during the mocks, so more practice on writing essays under timed conditions would have helped me. Also, ask people who got better marks than you in mocks to see their essays - work out what they're doing to get the marks that you dropped. You could get hold of a load of old mark schemes to find out if there are any points that examiners are always looking for, then regurgitate as necessary.
For French, Languages Online is seriously the best thing ever for grammar. Your exam board will have a list of all the vocab you need to know somewhere in their specification - as long as you know what all/most of the words in that list mean, you'll be as prepared as anyone. They can still ask you questions with unfamiliar vocabulary, but those words should be enough to work out the gist. Also, take time reading the question correctly - sometimes they phrase stuff weirdly to catch you out. If you get stuck on a multiple choice question, eliminate the stuff it can't be, before guessing as a last resort. If listening's your weak point, there are some videos on Bitesize, but they seem harder to follow than I remember GCSE French being (I did my French GCSE in 2011, so it might be forgetfulness).
For any subject, if you're still going on coursework/controlled assessments, aim for the highest marks you can get. Mark schemes change very little year-to-year on coursework, so take a moment to think about what to do to get that one more mark. It takes so much pressure off the final exams - I was very ill during my Year 11 exams, but excellent coursework grades brought my overall grades back up.