proper grammar and formatting of dialogue. Now I'll talk about the other aspects.
When you were younger, you were probably told to use words other than said in your dialogue tags (a tag is "he said", "she asked", etc.). You were probably told to go all out and use inquired, interjected, shouted, whispered, and a host of other assorted beasties.
Disregard this. Why? I'll show you.
"I heard what you said about me yesterday," Susan said, slamming her lunch tray down onto the table.
"What? I didn't say anything about you yesterday," Julie said.
"Don't lie to me," Susan snapped. "I heard it all."
"Yeah? Go on, then, what did I say?"
"You told everyone I'm in love with John Johnson, which I most certainly am not," Susan said.
This is a terrible example, but I hope it serves my purpose. When you read that snippet, you knew that Susan and Julie were trading lines back and forth. You knew this because, in all but one case, I used dialogue tags. Now go up and look it over again. Did you notice that I used said three times in the space of five lines? (Alright, maybe you did. If you need a better example of this, go and pick up any book, find some dialogue, and read carefully.)
Many times, your brain simply skips over the said. You registered the name of the speaker, but probably skimmed the said. In most cases, a simple said or asked is much more effective than something fancier. If you go overboard, your reader will start to pay more attention to the tag than the words being said, and then you're in trouble. For example, if you wrote "I think that's a horrible idea," she expostulated. it would sound ridiculous. Never let your dialogue tags take away from your dialogue.
The trouble is, you can't use said every single time, either. It would stick out and sound awkward. The trick is to find a happy medium. Use said often, but don't let it become the only tag you know how to use. Writing effective dialogue tags is something that takes practice and lots of trial-and-error (even I'm still working on it!), but you'll get there.
In many cases, you won't even need a tag at all. Often, your narration between dialogue will serve that purpose all on its own. Think of the last conversation you had. You probably didn't just sit there and talk. You (or the other person) ate something. You played with your keys or flipped your phone open and closed. You walked, ran, smiled, cried, put your hair into a ponytail, drove, etc.
Characters are the same way. As a general rule, they'll be doing something while they are talking. Which means that you'll have to put narration between dialogue. In these cases, you don't need a dialogue tag at all. Don't use one. Consider this:
"And just how do you think we'll accomplish that?" Fred asked, kicking a rock that was in his path.
There's a dialogue tag there. But is it really needed? No, it isn't. Here's the same line, without the tag:
"And just how do you think we'll accomplish that?" Fred kicked a rock that was in his path.
I took out the Fred asked, because we already know that Fred asked it. Hence the question mark. There's no reason for the tag, since I've got a line of narration in there anyway. This way, I eliminate an unneeded word and make it easier to read.
Yet another thing to consider while writing dialogue is that people have their own ways of speaking. If you listen closely to people, you'll notice they have words they favor. Some people use cliches like "It was raining cats and dogs". Some people use "well" or "so" quite often. A person that grew up in a household that spoke both Spanish and English will probably throw in a few Spanish words into their English, whether they realize it or not.
Take telling someone to open a door, for example:
Bob the businessman: "Open the door."
Grandma Agnes who bakes everyone cookies all the time: "Would you please be a doll and open the door?"
Bruce the school bully that you're stuck with for a group project: "Open the door, punk." (except that I've never heard someone use "punk" in real life like that. But oh well.)
The shy, non-confrontational person: Doesn't even ask you, just opens the door themselves.
The ex-boyfriend that you never want to speak to again, but you're trapped in the same room: Won't ask you either, just sends meaningful glances between you and the door in the hope that you'll get the message.
Each person has their own way of saying things. As a general rule (with some exceptions), you should not be able to take a snatch of dialogue from one character and give it to another, without having to make some changes first. Even your characters will have different speaking patterns, so no two should be alike.
The last thing I want to point out is how often writers use "hissed" as a dialogue tag. This makes no sense. Unless there is an S somewhere in that word or phrase, it is impossible to hiss it. Try hissing the sentence How are you today? It just doesn't work.
I think I covered everything. Again, here is a post on the proper grammar and formatting of dialogue. If you have any questions or comments, don't hesitate to let me know! If you want to know more, here is another excellent article about dialogue.