Make sure you organise your essays into a logical structure; know your facts inside-out; use good style and correct punctuation; make sure your answers don't stray off-topic; that's about all I can think of.
Meh when I was doing my AS history last year I kept getting Bs for all my essays. Then banged out a cheeky little 95/100 and 90/100 in the modules by just working harder during exam time. Though tbh with you I didn't work hard for those Bs. All I can say for history is remember all the little details first by writing out notes and stuff on paper and then condesning them onto flash cards and memorising them. Could work with your other subjects as well