I'd advise you to start past papers as early as possible and aim to finish a paper in 45min after some practice (core modules, applied takes longer in general), in an exam this leaves you 45min to go through every question, every line of working and make sure there isn't a mistake. Use your calculator (if allowed) and substitute numbers in to make sure your answer is correct, if it isn't then redo the question in rough until you find your mistake then copy out the new method. I find this works very well compared to spending the majority of the exam doing questions and having 10min at the end to check your answers as you can be in a rush if you find a mistake in the last few minutes of the exam.
This doesn't quite work for FP modules as they require a lot more work to be done for proof by induction etc and it can be hard to finish a paper that quickly unless it's fairly easy.
As for during the year, don't let yourself get behind in work, it is quite a lot to do and if there's a topic you don't understand it can be hard to move forward in the course as your next lesson will likely build off the previous.