Gain inspiration from your users by involving them in the design process. It may lead to unexpected and sensible project outcomes.
Organise creative workshops with end users or offer design tools allowing customers to create their own product. Consider adding a contest element in order to motivate participants.
A group of users who are willing to be involved in the design in a creative way.
Moderation skills to ensure and manage diversity and facilitate and encourage user creativity.
An ability to come up with creative exercises and design games to allow the user to bring his experience to the table.
Co-creation sessions are a default part of a context mapping exercise. Several companies have used co-creation in the marketplace by, for example, asking users to describe a recipe for a new flavour of potato chips or to design a sneaker.
The chapters on Generative research (ch44, p94) and creative toolkits (ch21, p48) in Universal Methods of Design give some input in how to do a co-creation.
The book Convivial Toolbox (Sanders & Stappers) gives much background and how-to information about co-creative sessions.