Like any skill, creativity needs to be exercised to keep it going. Even when you don't have a particular problem that needs your creative solution, keep working on your creativity. It will help you when you are suddenly confronted with something that needs thinking outside the box.
Alphabetize words. Take a word from a magazine or a billboard and alphabetize its letters. For example: the word NUMBER would be spelled B-E-M-N-R-U. The reason that this exercise boosts your brain functions is that it forces you to use all the information you're given (all the letters) and do something unusual with them. It trains your brain to come up with surprising connections and solutions and look at problems differently.
Make a game out of coming up with new or different uses for items in your house. This will teach you to look at items and situations with an eye for the unusual approach. For example: using an old boot as a planter, or building a table out of books.