I was once told that if your characters each had their own voices, things like "Said Sheila" or "John added angrily" would just disappear from your writing. I have found this (largely) to be true. I make all my characters have their own voice, by saying things that they would say and giving them habits of speech. Some use "uh" a lot, others have a lot of hesitations..." Here's some examples from my NaNo novel for 2010.
Warren is articulate, has a slight “east coast” accent and has a tendency toward long expository sentences – he speaks as though he is writing.
Akay has a sing-song tone – amerind/first nation (sounds like Adam Beach from windtalkers) . He tends to be wordy.
Martine is terse and growly
Bоўк has a habit of adding yes? No? to the end of sentences
Joyce (Bоўк’s wife) has a heavy Virginia southern accent
Alex has a terse manner, sometimes omits articles (a, an, the) and generally grunts if a grunt will do.
Marcia(Alex’ wife) has a southern accent but not as pronounced as joyce.
Justin has a “new yawk” accent and tends to be impatient. “C’mon” and “Gidda move on” come up a lot in his speech.
Rod uses “hmmm” before every sentence
Lee never answers without first asking “What?” even if you know he heard you perfectly