Dialect is best left at an impression of dialect.
What I mean is, give your readers the flavor of the dialect... not the whole 1500 calorie meal. A story that reads like this:
"Yer man done gone out an' had' 'im a goodun with that thar neighbor lady down yonder. Rollin' in the hay, squealin' like two hogs in heat. And he do know betta. That woman ain't but barely one as it is and him a full grown man with little'uns of his own. I dun thought he were gonna die of a heart attack 'afore they got dun."
... that's far too over done. Flavor your dialog with the dialect, never in the narrative. Especially in first person. People do not write the way they speak. If you read my forum posts then talked to me on the phone, you would never, ever equate my speaking voice with my AW handle. I'm about the biggest hick you'd ever hear - not Southern, not country... pure 100% hick. When writing dialect, be sure for one, that it all matches, for another, that you sprinkle it in. Don't write every word this way. The above passage is annoying. Read it again, and then read this next one.
"Your man's done gone and had a good lay with one of the neighbor ladies down yonder. Heard 'em squealing like hogs in heat, rolling around in the hay wagon like teens. At his age, he should know better. That woman is still mostly girl and he's a grown man with kids. I thought he was gonna have a heart-attack before they were done."
The one thing about dialect that most writers never get is that it's as much about how the sentences are constructed as it is the words they use. A southern style construction can flavor the words (if you are going for a southern impression, these are just examples) far better than writing out the word phonetically. Using certain words the same way every time they are encountered (say, for instance, using gonna instead of going to) helps. Most of all, please, please, please use dialect sparingly. With a deft hand. Flavor instead of over-power. Too much salt can ruin a good meal.