I'm sure most of you have read books where the author changed the spelling of words in order to express the pronunciation of a particular dialect. It used to be done all the time (Huckleberry Finn, A Little Princess etc.). Even now it can be done well, and even brilliantly (I think immediately of the dialects invented by Mike Flynn for The January Dancer and Up Jim River). However, if it isn't done right, it can be embarrassing, inconsistent or even incomprehensible.
This is why I don't do it. I still do dialects, though, so this article is about how to make dialects sound different without actually changing spelling to reflect pronunciation.
Fortunately, there is a lot more to dialect variation than pronunciation alone. There are also variations in pronoun usage, variations in syntax, variations in prosody (intonation and meter), variations in the use of the verb "be," and variations in vocabulary. Because I'm talking about writing in English, I'm going to stick to these - but it's good to be aware that in other languages, you can also have variation in other parameters (in Japanese, verb endings also vary by dialect!).
So let's do these one at a time, with some concrete examples. Pronouns (I/you/he/she/they/etc.) are a wonderful tool. Any change you make in the way you use them will be highly visible, because they resist change rather wonderfully (it's extremely difficult to get a reader's mind to accept a new made-up pronoun unless it resembles an existing pronoun very closely).
A great science fictional example of pronoun change comes from the work of Aliette de Bodard, who works with the Xuya Empire, a wonderful far-future version of the Chinese empire. In this universe, the Emperor is always referred to as "The Emperor ytself." I'm not sure about you, but the moment I see this I know that I'm looking at a genderless pronoun. There are two things working for me when I interpret this. One is that the pronoun would be pronounced just like the pronoun "itself." The second is that it has a very simple spelling change that tells my brain "look out!" This spelling change also leads me not to expect the default interpretation of "itself," i.e. that there is some kind of genderless object running the empire. There's a lot of mystery surrounding the person of the emperor here, but I don't immediately guess that the place is being run by some sort of machine.
I decided to change pronouns when I was designing the undercaste dialect of Varin, but in a more extensive way. These people start using plural pronouns for each other as soon as they reach adulthood. Now, surely most of you are familiar with the pronoun "y'all" from the American south. When I first learned it I thought it was used as a plural form of "you." Interestingly, though, at least in some regions it is a singular.
y'all = you (singular)
all y'all = all of you (plural)
This was a good thing, because I knew that the idea of pluralizing a pronoun wouldn't push people too far outside their comfort zones. However, I pluralized more than just the second person.
I => we
we => all-we
you => ye
you => all-ye
he/she => they
they => all-they
The result is extreme, but comprehensible once you get the hang of it. I was trying to make sure I introduced it in a very comprehensible context, so the first line that contains one of these pronouns is this:
"Give it to us, then."
Perhaps you notice the similarity to existing English dialects from the UK? This was fortuitous, but I'm ready to use it to the hilt, and you should be too, so remember this: the dialect you create may well evoke existing Earth dialects, and if it resembles one that bears some social similarities (casualness, lower-class) to the group you are working with in your world, this will really help your readers to get the picture.
Variations in syntax are cases when you change the order of words. For most of you, I'm guessing Yoda will leap to mind. He's weird (and possibly annoying) but he is comprehensible. One of his main strategies is to take the object of the sentence and promote it up to the front of the sentence, so that instead of Subject-verb-object, you get Object-subject-verb:
Your father he is.
Now, if you go in and start doing an analysis of everything Yoda says, you'll find he's not particularly systematic. However, when you're altering syntax for your dialect, I encourage you to be so. If you can stick to a particular pattern, then the learning and comprehension burden is reduced for your readers.
I did my own syntactic alterations when I was designing the alien voice for "Cold Words" (Analog, Oct. 2009), and I've analyzed it here on the blog, so I'll direct you to that article if you want lots of details about how it was done. That was a case of rendering an alien language in English, so it had a lot of different feature changes! [An Introduction to Aurrel]
Variations in prosody can be huge. This is intonation and stress, and all you have to do is choose words carefully and put them in a particular order to get it done. You don't have to change spellings, and you don't have to use special words. I have at least a couple of characters whose dialects are distinguished only by word and rhythmic patterning. Here is one example:
Pelismara (standard) dialect:
"You're all right now. How do you feel?"
Safe Harbor sea level dialect:
"Oh, young Master, sir, please tell us now you've not gone deaf or blind, and ease us all our worry?"
I shouldn't forget to mention "be." This is a verb that does a lot of helping but isn't very heavy on content, so perhaps that's why it ends up changing so much. Some dialects of English don't conjugate it at all. "I be going..." "They be good people..." etc. Change your default language on Facebook to "Pirate" and see what happens! This means that not only are people accustomed to seeing the word "be" used in variable ways (and thus will tolerate your alterations more easily) but that using the unconjugated "be" gives a very particular flavor to the dialect you're creating. This can definitely work to your advantage.
The next one to look at is changing vocabulary. In fact, if you're writing in another world, you're probably doing this already. Science fictional neologisms like viewport, commlink, etc. all would fall into this category, and so would created words for objects in fantasy worlds like "laran" psychic power in the Darkover world of Marion Zimmer Bradley and Deborah J. Ross. The thing to watch out for here is not to create so much new vocabulary that you're interfering with comprehension. SF neologisms have the advantage that very often they're pieces of existing words, like "mods" for modifications. However, if the context is not clear, they can also become confusing. One great thing you can do with vocabulary is create a sense of judgment and perspective. I've mentioned before that any object in a world will tend to be called different things by different people. A weapon used specifically by one group of people will tend to have the name of that group associated with it (in Varin, Arissen weapon or Imbati shot) - but only when being referred to by an outsider group. Arissen would never refer to their energy weapons as "Arissen weapons," because that wouldn't make any sense. They would have intimate knowledge of the variations in these weapons, and so would categorize them based on their function, as bolt shooters vs. arc zappers. Their familiarity with the types would show in the casualness of the terminology. We see similar things in our own world when we're looking at how laypeople versus clergy refer to objects having to do with the church, or how laypeople vs. medical practitioners refer to health issues.
As you can see, changes in vocabulary can hint about attitudes and culture within the group that uses those words. The terms we choose will have flavor, so as you make these alterations, think through which flavor it is you want to impart to the dialogue. If you want to go even further, you can think about how the usage of a particular dialect reflects historical developments, or cultural developments, in the community you're working with (the undercaste plural pronouns have a cultural and historical motivator, for example).
All this is just to say that if you restrict yourself from using spelling as a major tool in creating a dialect, you're really not "restricting" yourself much at all.
Now, go forth and have fun creating dialects!