Learn new things. The more you broaden your horizons the more connections your brain is going to be able to make. The more information your brain has access to, the more it's able to come up with unusual ideas.
Take classes outside of your field. This could be anything from cooking classes (provided you aren't a chef) or rock climbing. The novelist might then use what she learned in their cooking class to come up with a system of magic (the people who have a feel for what they're doing and don't use instructions as opposed to those who carefully follow a specific set of instructions).
Learn a new language. Not only does this help keep your mind sharp and forming new connections, it can open you up to new ways of thinking. The ad person might use it to start a section of bilingual ads that reach to a different group of people than she had normally been targeting.