Look to unlikely people as a source of fresh ideas. Many revolutionary inventions came from people outside the typical field of expertise. Invite ideas from the widest range of experience, education, social status and age. If you expect you'll find creativity there, chances are you will. Give employees experiences that are far removed from their usual activities.
Variation is the mother of creativity.
The more varied the experiences you can give people, the more creative they'll be. If one of your employees works primarily on banking accounts, give him or her a manufacturing one. Send the employee to a manufacturing plant. The further afield the experience, the more likely the person is to get some creative benefit. Don't discount chance.