I always use a phonic song "A for apple, A says Aah" - I don't know who sang it but my mum was an English teacher and so taught it to me when I was very little. I can't seem to find it anywhere but if there is a Japanese equivalent on youtube, it might be worth taking a look. Another thing I do is allow them to play with those toys that make noise - that say "Aah" when you touch A. Or since our class is techfriendly, we use the mini educational games
from Knowledge Adventure. The key, as with everything, is practice, practice, practice. Or in your case, it would be repetition.