I'm in a similar position with maths, not really down to having a bad teacher, but more because I'm nowhere near as quick to pick up stuff as the others in my class, so I had been getting behind quickly. :s
I'm starting a course which is meant to be an equivalent to AS maths this year at the college I'm going to, but is a lot like the higher maths so hopefully that should help, but apart from that I'd say just start as early as possible and try and do half an hour a night, and work your way through the different pages in a revision guide/proper thick question book, finding questions on the internet to do too.
Some people say past papers work better, but the above method's been working better for me. I suppose past papers work better if you're reinforcing knowledge you have already rather than trying to learn stuff you don't.
Damn, that's a lot! I'm not sure how many which is slightly concerning; after a quick count I reckon I'm taking 10 GCSEs, plus a BTEC and functional skills ICT, but I'm not sure how many exams per subject. :/ Also, are english lang. and lit. counted as separate and core and additional science? It is quite worrying how little I know about the courses I'm taking haha!