I don't know how good the resources are on TSR but if anyone knows, then you could subscribe to TSR and access them, i believe as a member you can also do other cool things that are helpful like study groups or something
When i get my own money i'm going to subscribe, it's one nice big community full of help
So i did AS Chemistry (got a bad grade so dropped it), i think the main reasons were i didn't revise early enough, i didn't do enough practice questions (do looads, and under timed conditions too), because even if i understood the content, the exam questions weren't as easy as knowing the content, also i reccommend you go to the site called 'chemguide' particularly for halogen topics, they have explained it 10 times better than the textbook! And a lot of other things too
For biology i basically write my notes in condensed form, but i create a margin on the right, in there you make questions out of all your notes, so you can test if you can recall everything
then just do the whole past papers stuff, read examiners reports too!
I haven't done maths, but i guess the big factor would be to understand the meaning behind the numbers, i heard somewhere it becomes wordier at A2? And of course practice