Listen to classical music. Some studies show that aerobic exercise increases oxygen and nutrients to the brain, thus improving mental functioning. Do habitual tasks in a new way. When you vary routines, you challenge your brain, too. Find a quiet place to work.
Distractions diminish ability to concentrate and think clearly.
Sleep on it. After working hard on a complex and challenging task, head for bed and let your mind work on it while you sleep. Friends tell me that meditation can accomplish the same thing. Look for the humor. Humor is great brain food because it tickles the mind to see things in a novel way.