Go to the stack of magazines and/or catalogues you've been hoarding (yes you do, we all do) and pull one at random. Leaf through it and pick and cut out two people. Just from the shoulders up. It can be two men, two women, one man and one woman, a woman and a child, etc. Choose the faces you're drawn to, go with your first instinct.
Lay those two people out on the floor or on your desk, sit in front of them, and start writing a dialogue between them. Don't hesitate and try to come up with something clever, go with the first thing that pops into your head and start writing. Any time you catch yourself trying to get a peek ahead down the road in the conversation, turn your head and look away! Focus on what's right in front of you. Imagine you are at a restaurant or in a department store or on a bus, eavesdropping on this conversation. Listen carefully and write exactly what you hear. This approach lets you off the hook as the 'creator' of the dialogue. You're just overhearing and taking it down. It has nothing to do with you. If it's going well, try to tune out that part of your mind that's saying happily, "Wow, this is going really well! This is the best writing I've done in a while!" And ESPECIALLY tune out any voice saying, "This sucks. This is stupid. Why would anyone find this interesting?"
Again: you're eavesdropping. You're not in charge of what's being said. Who knows what they'll come out with?
If you're having trouble focusing and/or tuning out your mind's commentary on the process and the writing, try one or all of these:
1) Imagine you're sitting in front of a blank, white (or black) wall. Of if you must have a color, pick a color. The whole time you're writing, stare at this wall, keep your focus on it to prevent your mind from voicing its opinion.
2) Go wash some dishes. Take a little voice recorder, turn it on, place it on the counter nearby, turn on the water and get scrubbing. The repetitive action will give that gregarious part of the mind something to focus on and let the other, creative part take over. Speak in the voices of these people (maybe announce who's speaking first, so you'll know when you listen later. For example: 'old woman,' or 'teenage girl').
3) Close your eyes and visualize the setting in which you're hearing the conversation. Really get into it. If you're on the bus, feel it's lurching motion, hear that diesel engine whine, the hiss of the doors, the squeak of the brakes, the way people have to raise their voices to be heard. Picture the people you've chosen. Imagine their gestures and expressions, their tone of voice. Imagine yourself trying to act like you're not listening and be subtle about writing down everything they say.
4) Just stare at the pictures you've cut out. When one of them is speaking, stare at his or her picture as you write what he/she says. When the other one cuts in, turn your focus on him/her. (This is a variation on #1.)
Once you've done this for about 10 minutes (yes, I said 10 minutes, it only feels like an eternity. Even if you have to stop and start several times, do it the full ten minutes), stop and put away your notebook, or save and close the file on the computer. If you used a voice recorder, turn it off and put it in a safe place. Do not try to turn it into anything. You can look at it in two days. You can wait longer, but not fewer. In two or more days you can open up the page or file, or listen to and transcribe the tape (for the latter, type out every word, no editing or leaving anything out), and then see what you have.
Remember this for all exercises or new writing: whatever you first write - put it away for a few days, do not try to edit it immediately. You might edit yourself right out of the best part of the piece because your first reaction was to toss it. You'll judge it too early, without any distance, and that can be a big mistake. Usually first reactions are based on what we think other people will like or want to read, or what will 'sell.' Who cares? You want to be the writer only you can be, not the writer everyone else is trying to be.
As with all the exercises I give you. This should be fun. If you don't get anything out of it that you can use - it's still a great way to practice writing dialogue.
Take it further: instead of imagining two people, find some real ones. Go to that restaurant or the coffee shop or the hair salon, take a notebook and a pen, and write down what you hear. These days you can hear some pretty surprising things. People talk on cell phones even in the bathroom stall now. Instead of being annoyed, tune in!