You're lucky I'm willing to help you with that clown staring at me! Or maybe I'm feeling obligated to help BECAUSE he's staring at me.

Anyway, so I only have a couple of tips. Write like you speak, but also, don't be too heavy on the accents and such so that it's too hard to read. But one thing about making it sound natural is not always using full and complete sentences. Yep, horrible! The grammar lady at the grammar table would disapprove, I'm sure. But seriously, we very often don't talk in complete sentences. Stop and listen to folks. REALLY pay attention. So, for example, this might be an exchange at a restaurant.

"I'm Nancy and I'll be your server tonight. What'll ya have?" (Some might debate that it should say "you," but I think it depends on how important an accent is, how much of it there will be, etc.)

"Number 4, please, extra mustard, no onions."

"Fries?"

"Sure."

Comin' right up." (Again, that apostrophe is debatable.)

See how they aren't exactly speaking in full sentences? But I think people often create full sentences where they wouldn't naturally appear. I do it all the time. But there is a fine line. You don't want to slow readers down with a bunch of accent stuff, but you also don't want everyone speaking in fully complete, grammatically accurate sentences because no one REALLY speaks like that. Like QueenNormaJean maybesnow?! said, try it out loud. That can often help. And definitely start paying attention to what other folks are saying and HOW they're saying it.