I cut and pasted this from my article, but here is my two cents (possibly two and a half). I may well be regurgitating what other posters have said already, but thought I'd post incase the examples etc helped anybody:
There are four very common mistakes that are made regarding dialogue tags:
1) "Said" is such a boring dialogue tag. I ought to whack out the old thesaurus, and I should weld on a zillion adverbs too!
As we say in England: I don't bloody think so.
Dialogue tags are not about winning the Nobel prize for literature. They are fuctional: they are there to tell us who said what and how they said it. I want you to think about this very closely: how much of that work has my dialogue already done for me? Has my exclamation mark implied a shout? Has the ranty slang implied my character's distaste? Your tags and adverbs should not distract from your dialogue.
Sometimes, said doesn't cut it. Sometimes your character really does need to gasp, rant, cry, sigh, whisper, mutter or grumble. Sometimes they might need to say the line quickly, softly or scathingly. Use tags and adverbs sparingly, though -- rare use lends a line far more impact and adverbs often disturb the rhythm of a sentence.
Be wary of tags and adverbs combined -- you don't need both 90% of the time. Be especially wary of combinations that mean the same thing: shouted loudly, whispered quietly.
2) Dialogue tags always go at the end of the line of speech.
If you follow the same pattern every time then your prose gets flat and lifeless. If every line ends in "she said," or "he sighed," then you've got that problem -- it's hard to inject any energy when it is read aloud because you're always adding on that low little "he said/she said/Tom cried."
Furthermore, not every chunk of dialogue is said in the same fashion: the character might smile half way through for impact, or break to sob. For example:
"I remember Grace, yeah. The girl with all that bouncy blond hair?" Emilia smiled. "She was so funny when she had to do that phone interview with Simon Cowell...
3) Every line of dialogue needs a tag, right?
No! And thrice no. I don't care if they're debating the Russian economy or masturbating to the dulcet tones of Celine Dion; do not tag every line of speech! Here's an example:
Annie thrust a menu on to Luke's lap.
"What do you want for dinner, hon?"
"Ooh. Let's have a looksy." He pushed his glasses back into place with a nail bitten finger. "Something with hot sauce, I think."
"Oh yeah. Anything for the sauce," she giggled.
"Are you able to say anything without innuendo?"
We already know that Annie is speaking because we described her action first. For the remainder of the conversation, we also know that the only two people speaking are Annie and Luke. That means that we don't need to be reminded every time that we speak. Did you even notice the lack of tagging there?
4) It's not enough to use a tag -- I need to say what my character is doing, too.
"Did you see the big game last night?" Rex called, striding towards my desk. "It was awesome!
Now look at:
"Did you see the big game last night?" Rex strode towards my desk. "It was awesome!"
Did we need to know that Rex was calling as well as striding? No: he was striding towards the desk and was therefore some distance away to begin with. His voice would have been loud enough to carry.
We don't always consciously think about these things when we write but we do often see mistakes like this in a story. When you re-read your work, ask yourself: do I need the tag and the action? Would the action act sufficiently as a tag too?