Year 11 not counting for anything should only be the fallback if you stuff up, not your general attitude...general advice from me would be, just study, do what is asked of you and do it well, make all your mistakes then, and that will lay the foundations for year 12. Stay on top of things, there isnt time to make up lost ground, thats more advice from me. Form good study habits!
More specifically....subject wise. I dont know what subjects you do, but here's my advice to all you prelim students for the subjects I did in any case.
For English, and this is very important...LEARN HOW TO WRITE. Learn about the sorts of writing styles needed to succeed in year 12...I found it would have been so much easier to find out how to write properly, how to set it out right, how to analyse quotes and technique, how to discuss ideas etc etc. It would have lifted my mark slightly. That way you wont be doing all that stuff in year 12, and you will be prepared. I was still learning all that stuff the day before the English HSC exam...
Maths, learn everything, because everything is culmulative, same goes for languages. Setting out properly is a must, systematic approach is imperative (sorry thats for maths). If you are doing a language, take time to learn the vocab, you will not get away with it if you dont. I know, I've tried. Dont miss anything, otherwise you will just fall behind and will always be behind, trying to catch up when there just isnt time. I learned this the hard way.This is most important for languages and maths, there is just no getting away with it.
Sciences, just follow the syllabus and make sure you know every dot point on that, same goes for PE.
And thats all the advice i can offer to you, subjects wise as I didnt do any other subjects. Chances are really that you wont heed this advice and learn what I learned the hard way. Or if you work hard and go well, well it wont be because you listened to this.
Whatever you do its all about habit, get into a good one. And don't take yaer 11 too seriously, its not worth it, all in all it counts for nothing (but still try, because it prepares you for year 12)...save that for year 12.