I always think a great way to learn about writing different accents and dialects is to read different authors who use those techniques. Like Roddy Doyle or Shani Mootoo. Of course, for me, half of that is research and half of it is just enjoying a good read. :P Wikipedia is also a surprisingly good resource. I see it's already been noted. :)

I started thinking on this subject when I was using Middle English spellings for a character's dialogue, to give the character what I thought would be a safe accent, because no one uses Middle English anymore, and the admin asked me to refrain, because she was worried it would insult someone. So now I'm always worried about doing that, but I also can't help giving in to the temptation to try writing things that way. I know the easy way out, and a fairly good way of avoiding this whole problem, is to just say the character has an accent from one language or another, but I like the challenge. Has anyone else encountered this problem? That writing out the accent or dialect is seen as (potentially) insulting?