hey - I just misread your answer and though you were AS - A2 not gcse-AS.
anyways. definitely read through the specification - check whether it is old or new. I would expect especially bio/chem to have the new spec now for all exam boards. double check the exam board which is used at your school so your not reading the wrong one. You can find the specification on the exam boards website.
possibly invest revision books for your subjects from whSmiths - or online since its cheaper
, again double check the exam board, also if you know anyone doing the same exam board ask to borrow their study book or ask for the name as they'll probably be using it...
On a side note, dont burn yourself out during summer - you need to RELAX until September - grant yourself at least 2 weeks of non study related activity.
try filling up your time with a part time job, or volunteering to fill up your time - many places will be looking for school leavers, agree pay isn't great but you will earn some dosh you can spend for holidays etc - your parents don't want to fork out for you over this 2 month holiday.
also, a final note i'd like to say it isn't necessary to have 4A* @ A2 - usually you drop a subject @ AS, so you'll have 3 full a levels and an AS Level, unless you do further maths, universities only ask for 3 A Levels, yes even Oxbridge, which is why they ask for A*'s in 3 b/c they ask for 3 subjects
can't get A* in year 12 either unfortunately
and finally GOOD LUCK