I did two GCSEs that involved exams last year so I'm pretty clued up on this whole revision thing.
In Maths I only got an A though I really wanted an A*, there was literally no way I could've got an A* since most of the hardest questions were just impossible to revise for... they were complex forms of things that you learn and you just need the initiative to be able to answer them.
In French I got an A*, which literally made all the work seem worthwhile. What was most effective for revising for my orals (which I got full marks on) was sitting outside and just remembering one sentence at a time until I could recite the whole topic without thinking about it. For the other exams it was much easier, I literally did no revision for them and they went well too so I don't know what I can say about revising.
I retook science module b1b after only getting a C and was delighted to see I got full marks. Basically, what it was about was just making notes on all the things I didn't know, but make sure that you say them out loud and actually absorb the information. Try to make connections between things and it helps if you understand everything. I'm taking two retakes next Friday and to be honest I'm petrified, I've revised loads and tried practice papers but sometimes practice papers just knock your confidence. I really don't recommend them unless you're doing a straightforward exam that won't take long.
For written exams it's important that you know everything. Don't ever hesitate to sit down and read through the whole of your revision guide, but make sure you don't feel uncomfortable, that you actually want to do it. Otherwise your brain just won't take in the information.
For mocks, teachers are especially helpful. My class was supposed to be doing English last year so we had already done a mock and our teacher is helping us to understand exactly where we went wrong and telling us exactly what to do. In History we're being set a practice question every week, this feels like a big stress to me but I know it'll help me to get at least a B in my mocks. If your teachers don't help you as much as you want them to, just take the initiative to do something about it.
Wow, I wrote a lot.