Be nice to your teachers and don't bad mouth them; they are there to supportive, helpful and caring when it come to your work and grades. If they are being unreasonable or horrible, complain to the head of the subject and get moved to another classroom; neither you nor them should be abused.
Do your homework, it's there to tell you...TELL YOU where you strength and weaknesses are, NOT the teacher, yeah the teacher was to see and mark it. But it's mainly for you as guidance, if you're stuck in a certain area IT SHOWING that you have a weak spot where you need to study/revise.
Study/Revise as early as possible. The earlier you study and revise on your content/weaknesses, the more likely you're going to pass your exams, yeah A-Levels/AS are a memory game, but you also have to understand what your studying and learning, so that if your asked to elaborate, you know that you can.
(Aimed at GCSE students moving to A-Levels)
Take responsibility. If you don't do the work, the homework, study or revise when your results come up on results day/mock exam papers take responsibility. The difference between colly and secondary school is that the teacher will not hunt up down, be fussed if you don't study/do homework and more. So if the grades are not what you want, then take responsibility of that.
Pick subjects that you will love/need for your uni course. Do thorough research on the uni subjects that you might want to do. If you don't know what you want to at uni, pick subjects that you will interested in or liked at GCSE. You will be doing this for two years and it will require a not of attention and concentration, so pick something where you will love to study/revise and do work for.
Uni Applications:
Do your Personal statement as early as possible, so that you can do as many drafts as possible so that you can perfect it as much as possible, till you send it off. Your selling yourself to the Univeristy so don't take it lightly.
Be honest and do a lot of research on the course/uni that you are looking at; you will need a valid reason to convince the unis why you want to do there and why you want to study that course.
Last but not least GOOD LUCK!!