identify your weaker subjects / topics. make sure you spend time
on it till you get it. also, discussing about things you don't
understand with your classmates can help becus it sets you to think
really carefully. for me, it really worked becus sometimes, silly
jokes when discussing helps me to remember the solution to a
particular question easily. and it really gets etched into your
memory.
for maths, practice makes perfect. my maths used to be b3 b4 kind.
it's only till the HOD for maths took us in sec4 and gave us tons
and tons of questions for homework everyday. that's how i got my a1
!
for sciences, get exposed to the questions that appear frequently
and make sure you remember the key points for the answers. when the
O's are approaching, try to finish up all the MCQs in the TYS becus
questions repeat themselves most of the time. most of the MCQs in
the O levels for bio last year were from the TYS.
one of our teachers told us to start preparing bit by bit, slowly
it gets more and more intense (like around prelims) and by the time
when you're near your O's, you wouldn't have much to worry about.
just revising main points and the mistakes you're prone to make.
the 'graph' for the amount of effort you put in is like a little
hill.
anyhows, i'm glad you're willing to start early. remember to
complete all your homework. this way, your revision is considered
half done. all the best ! 
