Have a clear objective. Trying to be creative without one won't get you very far.
Don't evaluate your ideas immediately. Write them all down, even if you think they're no good, and select the best at the end.
Remember there is no risk in coming up with ideas - only in implementing them. For example, to think about getting married is virtually risk-free; doing it, however, is another story.
Write down the opposite of any idea you have that seems mundane. This technique was used to come up with diagonal oil drilling, life assurance that pays out while you are still living and the stuffed crust pizza.
Imagine you are someone else - a celebrity, a colleague or a friend. Think about how they would address your objective.
Use external stimuli. They can help to jolt your mind out of linear thinking. Focus on a picture (or a smell, sound or word) that has no obvious connection to what you're thinking creatively about and write down anything that comes to mind.
Take a break if you feel stuck in a rut. Thinking creatively is a bit like trying to go to sleep - the harder you try, the less likely it is to happen. Your subconscious mind will be turning over thoughts, so let them rise to your consciousness in their own time.
Ask others for their views if you think you're getting lost. It's a myth that you need other people to help you to be creative, but they can help.
Find the child within you. We were all creative as children and we're all capable of being creative now. It's only self-limiting beliefs after years of being told 'that's wrong' or 'you can't it do that way' that stop us from thinking the unthinkable.