I have the same problem too! We haven't employed any foreign helper so my baby doesn't have much english exposure. I have been trying to speak to him in English whenever I was with him alone but am wondering if he will be confused if I speak in Cantonese sometimes (when we're with other people, grandparents, etc) and English at other times (when we're alone).
My boy goes to daycare during weekends and I'm planning to take him to English-speaking playgroups on Saturdays, but i guess only 45 mins of playgroup a week won't help much.
some dvd in eng.version, let her/ him enjoy the story and also you need stay with them!!
I also bought some story book with easy sentence or words, start from simple first!!
Use English every day. It doesn't matter if it is just a simple word or a long sentence. Use whenever you feel confident. No need to avoid Cantonese. Kids are very smart, they can tell. But of course, it takes longer time for them to pick up both languages and they start to talk later than the monolingual ones.