Hiya, I went from an E to a B in Biology and jumped 2 grades in my other 2 subjects in a few months, and what I did was do SO MANY past paper questions. Geography past papers will be on Physicsandmathstutor.com so do as many of those as you can, and then do them again. I can't much help with the other two subjects since I didn't do them, but I presume the same applies. For the essay subjects, get your teachers to mark the answers for you and get feedback. Even if you don't know the answers, have a go, and study the MARK SCHEMES. Learn what the examiners are asking of you; the language you need to use, which points count most for the different types of questions. It seems obvious, but that's what really helped me. Good luck and I hope you smash it!