1. Don't underestimate AS, it's easy to relax and think "it's fine, A2s are the important ones". A few of my friends took that approach, and this year they have loads more revision to do, so lots more stress, because of resits. It doesn't matter if you don't get straight As, just aim to get good enough grades that you don't need to resit any exams the next year.
2. If you need to take time off because of illness or whatever, catch up on missed work ASAP. It's so frustrating opening your file to revise and finding gaps in your notes.
3. Don't make yourself busy with too many extra-curricular activities or social things. The thought of turning down some party invites to do school work may suck, but think about it, you'll have plenty of time to go out and enjoy life when you're at uni or wherever you plan to go after your A levels. Why sacrifice that for the sake of a few parties now? Sure, take time out to relax a bit but don't overdo it.