1) Vocabulary - Each character should have their own vocabulary or pet phrases that separates them from the others
2) Verbosity - You should determine the length of each character's thoughts and speeches. You may not get some characters to shut up while others need prompting to say two words. Stereotypically (so it shouldn't be used much) grandmothers, women or old people go on and on while young teenage boys talk in grunts.
3) Velocity - The pace and rhythm of a characters speech and be altered. It's in how you arrange words, sentences and ideas. One could speak without pausing for breath (maybe take out some of the commas) or one can pause often (maybe use more '...') or one could stutter (st-st-stut-tter)
Here is a good example of distinctive voices (take from Emma by Jane Austen - a conversation between Emma and Mrs. Bates):
"My mother's deafness is very trifling you see--just nothing at all.
By only raising my voice, and saying any thing two or three times over,
she is sure to hear; but then she is used to my voice. But it is very
remarkable that she should always hear Jane better than she does me.
Jane speaks so distinct! However, she will not find her grandmama
at all deafer than she was two years ago; which is saying a great
deal at my mother's time of life--and it really is full two years,
you know, since she was here. We never were so long without seeing
her before, and as I was telling Mrs. Cole, we shall hardly know
how to make enough of her now."
"Are you expecting Miss Fairfax here soon?"
"Oh yes; next week."
"Indeed!--that must be a very great pleasure."
"Thank you. You are very kind. Yes, next week. Every body is
so surprized; and every body says the same obliging things. I am
sure she will be as happy to see her friends at Highbury, as they
can be to see her. Yes, Friday or Saturday; she cannot say which,
because Colonel Campbell will be wanting the carriage himself one
of those days. So very good of them to send her the whole way!
But they always do, you know. Oh yes, Friday or Saturday next.
That is what she writes about. That is the reason of her writing out
of rule, as we call it; for, in the common course, we should not have
heard from her before next Tuesday or Wednesday."
"Yes, so I imagined. I was afraid there could be little chance
of my hearing any thing of Miss Fairfax to-day."
"So obliging of you! No, we should not have heard, if it had not
been for this particular circumstance, of her being to come here
so soon. My mother is so delighted!--for she is to be three months
with us at least. Three months, she says so, positively, as I
am going to have the pleasure of reading to you. The case is,
you see, that the Campbells are going to Ireland. Mrs. Dixon has
persuaded her father and mother to come over and see her directly.
They had not intended to go over till the summer, but she is so
impatient to see them again--for till she married, last October,
she was never away from them so much as a week, which must make
it very strange to be in different kingdoms, I was going to say,
but however different countries, and so she wrote a very urgent letter
to her mother--or her father, I declare I do not know which it was,
but we shall see presently in Jane's letter--wrote in Mr. Dixon's
name as well as her own, to press their coming over directly,
and they would give them the meeting in Dublin, and take them back
to their country seat, Baly-craig, a beautiful place, I fancy.
Jane has heard a great deal of its beauty; from Mr. Dixon, I mean--
I do not know that she ever heard about it from any body else;
but it was very natural, you know, that he should like to speak
of his own place while he was paying his addresses--and as Jane used
to be very often walking out with them--for Colonel and Mrs. Campbell
were very particular about their daughter's not walking out
often with only Mr. Dixon, for which I do not at all blame them;
of course she heard every thing he might be telling Miss Campbell
about his own home in Ireland; and I think she wrote us word
that he had shewn them some drawings of the place, views that he
had taken himself. He is a most amiable, charming young man,
I believe. Jane was quite longing to go to Ireland, from his account