Pay attention to other people’s ideas. Ideas don't exist in a vacuum. Even creative thinkers like Salvador Dali (as an example) started with ideas in his painting that he'd gotten from earlier sources. Paying attention to other people's ideas will help foster your own.
You’ll see how other people think outside the box. Learning other people's thought patterns and ways of thinking will help you keep from stagnating in your own thinking. You could even say to yourself "How might my creative painter friend view this problem of ads?"
You can also look at the ideas of famous innovators. Examine what ideas they had that worked and what ideas didn't work. Look at their practices for encouraging creative thinking (like the examples of Steve Jobs, Tchaikovsky, and Toni Morrison in the first part of the article) and try those practices out.