I know a lot of the advice given regarding this to do a lot of practice papers, and that’s very important! However I have found when it comes to things like Chem and physics, having your basics down is very important. I’m doing IAL as well and say for example if you’re doing organic chemistry, the lessons which you learnt at o levels will be the building block for your a levels. Similarly, what you learn during the first year is crucial for the second year.
To prevent forgetting certain material make sure to revise often and a little at a time. It also helps to have very brief notes just so you have an overview of the material if you want to refer when doing papers etc. Good luck!