The best advice I ever received about dialogue was from my first creative writing professor. He said that you have to hear dialogue, really listen to the rhythms and cadences of speech to effectively write it.
What's the best way to do that? Go somewhere you can listen to real conversations. Listen carefully. Listen to different types of exchanges. Parent, child interactions. Conversations between teenagers. Do they finish their sentences or do they truncate them?
Once you get the feel of multiple speeds and rhythms then it is time to start writing them. Sit or stand close enough that you can write down what words they use. How often do they repeat themselves? Do they take turns, going back and forth or is it less predictable?There are words that have developed from texting that have made their way into everyday conversation. Do you know what they are and what they mean?
By doing this often enough, writing conversation will become much easier.You'll start hearing your characters conversations in your head and effectively write it so it sounds authentic. I have pages of conversation that I have recorded. I've learned that it is really rare for a sentence to be more than 6-7 words. Usually, if someone goes over that the other person will inevitably interrupt. Language evolves and changes quickly and if we want to stay abreast of how people talk we need to always be listening.
Have you ever eavesdropped on a conversation? What did you learn? Has it helped improve dialogue in your writing? How?