1. Go outside and listen to people talking. Restaurants, coffee shops, shopping malls and sporting events are all great places to see how people interact naturally. Make note of their emotional states and patterns of their speech. Do they pause at certain points? What words make them slow down or react?
2. Decide where your character is from and then seek out material from those locations. Youtube is a great resource for this. If it’s a language you know, listen to the radio stations for that area. If it’s a completely new place (a fantasy land or foreign planet) get a few ideas of what earmarks their local language may have.
3. Read your dialogue out loud. With others, if possible. You can usually tell immediately if something sounds contrived or unnatural. Is the emotional force of the scene being communicated in the words, or is the conversation too flaccid? If you can, try to improvise with people and record what works.
4. Learn how to format dialogue in prose. This may sound like a ‘duh’ but I can’t tell you how many issues I’ve seen that have been caused by lack of clarity resulting from poor dialogue tagging and misinterpreted writing.
5. Tap into how you feel. When two lovers are talking to each other, do you get warm fuzzies? Do you get nervous when the hero and the villain are at each other’s throats? Do you get teary when characters are saying goodbye for the last time? Even if you aren’t having a dramatic response, your heart should have some sort of reaction to your writing. If it’s not, ask yourself why.