Become a master spy. Part of what you put in your notebook are your observations of others. Picture this scenario: You're having lunch, listening to your iPod in a busy restaurant, and writing in your notebook. What if the iPod wasn't turned on? You could listen to all the conversations around you, but it wouldn't look like you were listening. You're in a public place within earshot of all these conversations, so why not? Is it ethical? That's up to you to decide. But it is a great way to get a sense of dialogue and really hear the way people speak. Is there a customer giving the waiter a hard time? That could be a sketch. Is someone talking loudly on his cell phone? The other side of the conversation, the side you can't hear, might be a sketch. Here's a conversation overheard while waiting in line at a bank:
Bob: So what are you doing tonight?
Steve (frustrated): I have to go see Karen.
Bob: Oh, I don't envy you. Good luck with that.
Steve (letting it all out): Tell me about it. She is the most selfish person I have ever known. It's all about her. I just have to sit there and listen to her cry about her problems. She is so needy! She never asks about my problems, it's just all about her!
Wow! Why are they friends with her? If she's that needy, why do they stay with her?
What if Karen were an infant? That would explain it all — the crying, only caring about her own needs. There's a sketch. You have the conversation, then reveal that Karen is three months old.
Make sure to date your notebook when you start one, and date it again when it's full. This will help you if there is ever a dispute over the ownership of a joke; it will give you a range of time that tells you when you came up with the idea. It might even show you the context of the origin of the idea.
Get out there and listen. See if you can get ideas from conversations. If you feel weird about it, try this: Look at people who are way out of earshot and try to imagine what they are saying. Or turn on the TV, turn down the sound, and try putting your words in the characters' mouths. Or watch a TV show or a movie in a language you don't understand and try to make up your own story.