Hi,
A' Levels are a different ball game to GCSEs, if you're targeting A*s, then you need to keep up. My advice is make notes on each subject as you learn it, for example if you're a biologist and are learning about the PCR, make notes on it the night after the lesson. Go over these notes ever so often.
For the lead up to exams, past papers are your best friend. Remember, the work is hard, but at least you won't be kicking yourself come results day.
Good Luck with your A' Levels, I'm sure you'll smash them!