Ready for another tip?
Dialogue is what your characters say. Dialect is how they say it.
There are several things to keep in mind when you're talking about dialect.
The first thing is to remember that dialect gives a character flavor. When characters have flavor, they're easier to remember. Understand this: Gambit is a bad character, but he overcomes his several faults because of his dialect. His powers are okay, but his dialect is what gets him over the hump.
The next thing is that dialect has to be maintained. Too little and it can easily be forgotten; too much and it can be incomprehensible, or worse, annoying. The worst thing you can do to a character is make the reader want to punch them in the face every time they speak. This is something that can easily happen.
The final thing about dialect is to understand that it is regional. Someone from New York should sound different from someone from Alabama; someone from France should sound different from someone from Germany.
When you're doing a dialect, it isn't just about verbiage. Some words or phrases are regional, but don't think you can just slam in a foreign word or three and you have a dialect. You can also have to sound things out and spell them accordingly. This means you have to do a little work in order to do it right--which means you're also causing the reader to do some work to comprehend what the character is saying. (This is where incomprehensibility and reader-rage can come in.)
Dialect is sloppy. By that, I mean it doesn't have to be "correct" English. As a matter of fact, if you try to make it correct, you'll lose the audience (especially if the character is supposed to be from the place that the reader is from). Don't lose your audience. Don't try to be overly correct.
And there's your tip!