Other people have probably already said all this, but, start early. It is never too soon to begin making revision resources or doing past papers. Work throughout the year with the aim of always being ahead of what you're doing in lessons. Secondly, it probably sounds obvious, but be organised. Know exactly what notes are for each topic in all of your subjects; have all your resources/past papers/etc ready for whenever you need them. Lastly, if you're lucky like I have been and you have amazing, supportive teachers, use them. Let them help you, and don't be afraid to go ask them for help/advice or even extra work.
In short, AS year will fly by, so just try to keep on top of things and most importantly, enjoy learning!