As a cartoon gag writer, I spend the majority of my time coming up with gag ideas. To generate ideas, one of the things I do is a lot of reading: newspapers, magazines, online news services and so on. For example, if I'm writing gags about shopping, spending money, saving money, etc., I'll read books or articles about how to save money when shopping, or tips for living below your means.
But, what about personal experience? More and more that's what I'm writing from. Lately, looking at myself, the people around me and the situations I find myself in, is where I get more of my ideas. In order to write gags, you have to be able to see the humor in yourself and those around you. You have to have a skewed way at looking at things. More and more I look around at what's going on around me...the people I interact with, what's on the news, what's going on at the supermarket, in the parking lot, at the library (a great source of humor!), at restaurants (another great source!) and just write down what's going on and what people are saying and doing. Hopefully, others can relate to it.
People ask me where I come up with ideas. Lately I tell them I look at myself.