hmm...if it's for those characters, I'd take hints from canon, and write it the way JKR does, but if it's an OC or the author is having trouble with the phoneticy stuff, I'd say just write normally, but keep the character's culture and speech patterns in mind at all times. Sometimes a simple turn of phrase common to the area they're from is enough, or having one of the other characters mentioning how that person speaks may be enough to convince the readers.

Also, if someone is sold that phonetics is the way to go, my gut feeling would be to keep that character's sentences short, and not have them giving any huge speeches. Even when done well, a reader might tire of reading a long section written that way.

If all esle fails, my advice is to opt for 'less is more'. There's nothing that makes me cringe worse than an over-done accent. *shiver*

Hope that helps. *loves*