I think all the previous advice is very sound, with respect to being diligent and doing all the problems set. I think monkeyvirus gave some fantastic advice when she said , "figure out how you learn if you don't know all ready (visual, auditory or tactile) and use strategies devised for that sort of learning". For my part I always skim read through material seeking our any patterns or reoccurring themes and then I would go through it again, looking to attach physical and/or mathematical meaning to the text, however that method may not be for everyone. As you progress and mature you learn to hone and alter your style to become an altogether better mathematician.
I've always believed that all the individuals who have been permitted entry into a course have the ability to obtain a first. Learning style, discipline, focus etc.. even more mundane aspects such as getting enough sleep, eating right and staying calm will all help your mathematical ability. It's your job to tinker and experiment to find out how your mind works.
"How do the 3 years (say) compare ? Do you get a 1st in all years , 2nd and 3rd, or leave the majority to the 3rd ?"
For my part, I obtained better results in the latter years. Doing a 4 year course my averages were (63,60,70,81), which isn't quite a first with a 1: 4 :4 : 4 weighting of the years, but in borderline cases, precedent is given to the latter years. Throughout I would say, on average. I attended just over 1/2 of the lectures and a 1/10 of the problems classes. I preferred working by myself or with a small group of friends, where we would get together and discuss the topics (this happened more so in the last two years).
Anyway to summarise there's no substitute for hard work but in addition tailor your working style to your mind.