its a cliche but true: work smarter not harder
i.ei its not about the hours you do, its about how you spend those hours. 1 hour of 100% concentration> 5 hours of just sitting there.
This.
Plus working consistently. I know far too many people who cram it all in last minute, and don't get the results they desire. It's best to work throughout the entirety of the year (small bursts of study, regularly) rather than hard core cramming last minute before examinations. [Though I will admit, I still do the latter, force of habit; study more when the exam approaches.] Also I find being organised and prepared is very effective, getting concise study notes and material prepared in advanced to your exams decreases the stress as the exam approaches, as you can focus on revising rather than note taking.
It's best to work out what type of learner you are. Some learn from studying 4+ hours a day by reading + writing notes and summaries, others learn by creating visuals (those more right-sided of the brain), while others cannot concentrate for more than a short burst of time and thus must work at increasing that gradually.
Anyways best of luck. Best to trial out a few different techniques, and then work out which way you retain the more information effectively. Good luck. n_n