I think one of the most important things to remember about Modern is that it is a very content heavy subject, so to do well there is really no getting around the fact that you need to know your syllabuses (syllabi?) back to front. Even for WWI, where you only need a superficial knowledge compared to your other topics, there is still a lot of information you need to be comfortable with, because even though you only need enough to analyse sources sometimes they can give you really obscure sources or specific questions which requires depth of knowledge.
Obviously, for the other three areas your knowledge really important, so you have to be constantly reading and writing notes, going over them, discussing with other students etc. Like I said, there should be no area of the syllbus where you go 'I would just avoid that question', because if there are two of them then you are screwed. This depth of knowledge is extremely important because without it you can't develop your own opinion, which is the most important thing when it comes to essay writing. If you don't have strong opinions on everything in your syllabus, you will find it very difficult to respond to some questions. This is particularly critical for the Personality section, when they have some obscure quote the only way you can respond is if you know your subject matter so well it just clicks with your own views.
Opinions are also really important because they can help you develop interesting arguments when writing essays. Like, most people can easily regurgitate facts and basic theses from text books (which I think enoilgam covers in his amazing guide) but what examiners are interested in is YOUR view, and if you have a unique one then it is is more interesting to read and there is more of a sense that you have read/know more in order to formulate such a view. Essays, I think, are good on the basis of two things; complexity and clarity. Complexity is how interesting your ideas are to read (as I just said) while clarity is how easy it is to read; just simple stuff like topic sentences, clear paragraphs etc are critical to good essay writing
So to sum up, to do well you need to know a lot in order to be dynamic in responding to tricky questions, as well as developing your own focused opinions, which you NEED in order to do well.
Good luck!