I had a A* equivalent in a GCSE equivalent, and have given private lessons in Latin for 5 years now, so see if that helps:
Grammar: Repetitio est mater studiorum. You HAVE to know all forms by heart in order to deal with texts, and you have to come a point some time when you don't have to think about the Grammar any more. So keep repeating them. Try for example to draw one large grid in which you write all tempi for one word, (e.g. one for amare, one for videre, one for audire etc), then one for all the casus etc. It's a lot of work, but it works, really. Also, you can still look at them later for refreshing.
Prose: As was said before, reading the texts aloud is always a good advice. Also, read secondary materials. Have a look at English translations and try to understand how they were translated, the process etc.
Vocabulary: as in any other language, file cards are always a good idea. Or you draw some mind-map for one topic, for example Mythology or Sports or whatever, and sort the vocabulary in. Or write little stories (in English) and insert the Latin words you need to know.