1) Write like you talk. Scene: Two friends meet on the street.
"Hello, John. How are you?"
"I am just fine. And how are you, Martha?"
"Thank you for asking. I am fine. How's your job search coming along."
"My job search is going rather slowly, I'm afraid. Are you still working at Smith and Sons?"
Wasn't that painful to read? The characters are lifeless. You would never speak in that manner in real life, so please don't ever have your characters do it.

Here's a more likely scenario:
"Hey, John, what's up?" Martha asked.
"Not much, you?"
"Same old. You still out job hunting?"
"Yeah, sorry to say. You still at Smith & Sons?"
Now add some filler (deeper POV):
"Hey, John, what's up?" Martha's attempted smile never reached her pretty green eyes. She bit her bottom lip.
John considered his friend for a moment then heaved the heavy backpack off his shoulders. It hit the ground with a thud. "Not much, you?"
"Same old." Martha's voice was so low he had to step closer to hear. "You still out job hunting?" she said as she studied the sidewalk and ran the toe of her shoe along a crack.
"Yeah, sorry to say." He gently lifted her chin to peer into her face. Tears were about to spill there. He felt the pull of her chin as she tried to turn away. "You still at Smith and Sons or did something happen?"
Now we have two living, breathing characters.

2) Make it realistic. People often cut each other off.
"Did you put gas in the car like I asked --"
"Of course. Don't I always," he said, the irritation clear in his voice.
"I know, but tonight --"
He held up a hand to stop her. "I don't want a lecture."

3) Get to the point. Make if flow, but stick to the subject. In real life we tend to ramble, stop in the middle and start a new subject, or just not talk in complete sentences. This doesn't work well in novel dialogue. Limit the uhs and ums.
"Frank bought me the cutest card the other - oh, that reminds me, we need to order the flowers. Did Jennifer RSVP by the way?"
Although this was an actual conversation I overheard at lunch, it's too confusing and thrown together for a novel.

4) We speak differently with different people. What you say to your sweetie won't work with the boss. Keep your dialogue genuine to the situation.A man softens his voice when speaking on the phone to his girlfriend. A woman speaks louder and acts more brazen with 'the girls' than with her mother.

5) More body language, less words. Showing instead of telling creates deeper POV and makes the scene more interesting. As we are speaking,remember we are also doing something; moving around, drinking coffee, picking lint off our clothes. You're not just standing there staring at the speaker. Sometimes you don't need any dialogue to convey the message:
"I'm quite aggravated," she said.
OR: She crossed her arms tightly over her chest, tilted one hip, and gave him a raised eyebrow.

6) Keep your character's dialogue in sync with their personality. A nun probably wouldn't curse. A guy in prison wouldn't call PJs jammies. Be true to your character in word, tone and mannerism. A painfully shy man doesn't suddenly become a ladies man.

7) Accents and foreign language. If your character is foreign and has an accent, give the reader a few words in his/her language, but don't overwhelm with thick accents that are hard to understand or tons of foreign words that have to be looked up. The unwritten rule is if you must use the accent/language then limit it to two or three words per page. Git me drift, luv? All good, dahlin'? Och, lassie, ye ken? Ya mus can do dat ting, yah?

8) Don't over inform. When people talk they generally know each other. A person would never say:
"Tom, my old friend from grad school, it's so good to see you away from your three-bedroom condo on the ocean where you live with your two poodle dogs, Bridget and Sparky."
Never use dialogue as a filler or to increase your word count. Make it relevant to the story. Only use dialogue to move the scene along. Cut any useless conversations. They slow down the pacing

9) Dialogue tags. Nothing is worse than a long unending conversation where you have to go back and figure out who is saying what. Tags are not needed after every line though. An occasional 'he said' will do the trick and actually gets ignored by the reader and will not interrupt the flow. Characters say their lines. They don't cluck, snort, shout, retort, purr, snigger, interject or bark. (By the way, those are all 'telling' not 'showing'.) If the couple is having a fight, the dialogue can be a quick back and forth.
"I never wanted to go out with you in the first place," she shouted.
"But you did," he said.
"Only because you were so pathetic," she sneered.
"I only seemed pathetic because you were such a brat," he snorted.
"A brat? You're calling me a brat?" she said.
"I call as I see 'em," he retorted.
"Oh, yeah, well see this," she yelled.
See how all those tags get annoying? If you removed all the tags except the first one, you could still follow along easily. But, as this is a fight, a few POV fillers would fill out the piece nicely. Notice how I didn't always begin with the dialogue. "I never wanted to go out with you in the first place," she shouted, her foot angrily tapping a cadence on the tile floor.
His face began to darken with anger. "But you did."
"Only because you were so pathetic." She looked around for something to throw.
"I only seemed pathetic because you were such a brat." He jabbed a finger at her.
Her eyes widened in disbelief. "A brat? You're calling me a brat?"
"I call as I see 'em."
"Oh, yeah, well see this," she said, as she hurled a lamp at his head.
10) Men versus women. A female writer will often write the way she herself speaks, which will not work on a male character. Men and women think and talk differently. Men speak in shorter, clipped sentences. A man would never describe the pink color as salmon or peach. Men are more visual and prone to action; women have a better sense of taste and smell, and tend to analyze.