The other responses are good, but I'd recommend reading a book about improv comedy[0] (or better yet, trying it out yourself). They've got a lot of specific techniques that are meant to stretch your creative muscles. I did a bit of improv when I was younger and I still use those techniques all the time twenty years later, because they work!

Reading about the exercises is interesting but unless you actually try them out they are just a curiosity. I think the muscle analogy really fits when it comes to creativity.

[0]I like "The Improv Handbook" by Tom Salinsky and Deborah Frances-White