Dialect
When I first started writing, I didn't pay too much attention to the way people spoke. I figured I was lucky to get my characters to sound as if they were holding a real conversation, rather than reading alternate paragraphs from an 18th century tome on rhetorical devices.
Slowly but surely, I got better at making my characters' speech sound more natural. At first, they all still sounded pretty much the same, but after a while the voices started to be more individual. The process was both gradual and exaggerated - early on, I had one or two characters per book who had strong, unmistakable voices (nobody would confuse Telemain or Amberglas with any other character in Talking to Dragons or The Seven Towers, respectively), but everyone else still used the same speech patterns.
As I worked on it, I got better at making more subtle distinctions between my characters' speech patterns. A lot of it was instinct - as I got more sensitive to distinctions in speech, a particular line would "feel wrong" for a particular character until I rephrased it. And then I hit dialect.
Dialect, according to the dictionary definition, is a variation on standard speech that has its own grammar, vocabulary, or pronunciation. It's the pronunciation part that drives writers (and sometimes readers) to distraction.
Pronunciation is an integral part of speech, and it's especially important for dialect. Yet non-standard pronunciation is really difficult to render on the page (unless you use the International Phonetic Alphabet, which few readers are familiar with). Oh, there are a few things that work pretty well - a character who drops the final "g" or initial "h" on words like "writin'" or "'ospital" isn't hard to show. But it quickly gets murky after that. Phrases like "Whatcha doing?" and "kinda hard" work on the page, but they get very old very quickly. If you use them with too heavy a hand, they can really turn off a lot of readers, even if they are the only non-standard speech your characters use. And when you get to full-blown phonetic respellings like "I wad be laith to rin an' chase thee/ Wi' murd'ring pattle!" (from Robert Burns' poem), they can be practically unintelligible.
The thing people forget is that, like everything else in writing, dialect is mostly illusion. It's important that it be convincing, not that it be an accurate reproduction in every aspect and at all times.
Phonetic dialect has actually got two strikes against it: the difficulty readers have in reading it (see Burns, above) and the fact that different readers will "decode" the phonetic dialect in different ways, no matter how hard you try to make it clear. People speak with various regional accents, and any respelling is going to be filtered through those accents. For someone who speaks with a Southern accent, "lakh" is not a phonetic respelling of "like;" "like" is how you spell that word that Northerners pronounce "lyke." At best, a phonetic rendition of a Southern accent is not going to work for them; at worst, they'll find it actively insulting. And it is generally a very bad idea to insult a sizeable chunk of one's potential readership.
I did a bunch of experimenting and came to the conclusion that by and large, dialect works best on the page when I use non-standard syntax and sentence structure, rather than trying to respell it. Nobody had any doubt that my character Renee D'Auber was a Frenchwoman with a noticeable accent, yet she does not speak one word of French or have one bit of phonetically respelled dialog anywhere in either of the two books she appears in. Huckleberry Finn speaks with a pronounced dialect, but only about one word per page is respelled (and since the book is first-person, everything in it counts as dialog for these purposes). Manny in THE MOON IS A HARSH MISTRESS likewise speaks in an odd syntax, but not in a respelled one. Keith Laumer was a master at this technique - every one of the many alien races in the Retief stories uses a different scrambled syntax. They end up all being clearly and obviously aliens speaking an alien language, yet their dialog is seldom hard for the reader to understand.
There are, of course, exceptions. One of the more obviously useful ones is if you have a minor character whose accent is so thick that neither the viewpoint character nor the reader is supposed to understand what he's saying without paying careful attention. Some writers even play with this if the minor character starts recurring regularly; they'll lighten up on the respelling as the viewpoint character gets more used to interpreting the accent, but they keep the syntax scrambled as a reminder that the character is speaking with an accent.
