read up on your maths and physics!! in my oxford interviews, the questions they asked me were purely that. one interview was all about a yo-yo and calculating its energy (rotational energy etc) and the other interview was entirely mathematical. i had to draw the graph of e^(-x).sin(x) which doesn't seem so hard now, but when youre under pressure, it really helps to know your stuff so you have to "think less".
yeah, some extra curricular reading is really helpful, and try and get a bit of work experience too if you can. but you might also like to do some non subject related reading - philosophy for example, to demonstrate an analytical mind, which they look for.