like what my English teacher used to say, though it sounds stupid, reading fairytales story books do help. because they use simple English. Read it more than once. You'll naturally catch the grammer and all. Then you can go on to newspaper all and blahblahblah VOILA!

of course, speaking if more regularly, helps too. and i do agree that, if you used to talk to your friends in Mandarin, it's a bit tad weird to speak English out of a sudden. Ask them to help, for you want to improve ;p

And don't be afraid to make mistakes, because we learn from mistakes.