Like I said
the best way to improve it is to read a lot of good proper english. Read some novels. They are both entertaining and educational (at least, they are if they're actually good novels). Read some Heinlen: his English is impeccable and he's a good storyteller. LOtR is another good one for high entertainment value and great English. Go see some Shakespeare; while it's not the same as Modern English, it's fantastic English, and when performed well his plays are amazing. Go get some Branagh and some Olivier productions of Shakespeare on film; I really liked Olivier's production of Henry V.
When I was learning French, I read French newspapers (Le Journal de Montréal, La Presse, sometimes Le Devoir) every day. It was very slow going at first, but after a year or two I could read almost everything I wanted to at quite a decent clip. It also helped my verbal comprehension a lot, and even my ability to speak because I was getting a handle on how verbs worked.
