It's just a case of knowing your stuff.
Do your reading BEFORE the lecture, so that way, when you're in the lecture you're consolidating your knowledge, not learning everything for the first time. Print out the lecture slides beforehand if you can, so you don't waste your time copying out the slides, and can instead make notes on the teacher's comments or your own analyses.
They often give you reading lists, printouts and questions just for each seminar that no one ever touches. Touch them!! For every seminar, we had a list of 4 or 5 essay questions, and I usually tried to answer them before the seminar. I realise that you're SUPPOSED to do this. But no one actually does and it will really benefit you.
Don't skip things, once you get into the habit it's hard to get out of it!
Try and at least acquaint yourself with the library early on.
Read a lot of academic literature on your subject: why? So you can see how academics write. I was looking back on my old essays (from GCSE to A level to Undergrad) and I can see there is a progression of "child's essay" to "pretentious child's essay" to "actual academic writing". Some of the undergrad essays I've seen are written like you're still at GCSE, the language is so simplistic, and they're just repeating what they've learnt in the lecture, not actually analysing or being critical about it.
Rather than only reading the core readings. Try and read something from the wider, "suggested reading list" or even something not on the reading list, for each topic, sometimes you'll find an author who has said something BLOODY GENIUS and it will really help you out, you'll wonder how you would have coped if you'd only stuck to the reading list!
But to be honest, I still have no idea, last year I handed in two essays I thought were equal, one scraped a 62, the other was an 89. I'm starting to consider it might be about choosing topics your marker is interested in!!