According to John Cleese, here's what you need to be creative:
1. Space and time. Find somewhere quiet and spend about 90 minutes there "gently resting your mind" against the problem at hand. Push through the busyness that your mind will create and let it relax into an aimless playfulness.
2. More time. Don't pressure yourself to come up with an answer to the problem right now or as quickly as possible. Defer the decision until the absolute last moment it can be made in order to give yourself maximum pondering time and therefore gain the most creative solution.
3. Decisiveness. Once you've made a decision, stick with it, and when you're in that open, playful mode, don't shut anything out as being wrong.
4. Humour. Allow humour to keep bringing you back to your open mode of idea exploration, because it's the quickest way you'll get back there. If necessary, use absurd juxtapositions of ideas as stepping stones to potential correct solutions to be discovered later.
Mokalus of Borg
PS - The full video, which I found to be entertaining and informative, can be found here.
PPS - Obviously he explains his points in more detail and humour than I can go into here.