Flexibility does not often come naturally to people. Some may be predisposed to being loose while others are rigid or taut. Stretching or exercising daily is one way to train your body to be flexible; reading regularly is a good way to stretch your mind. For me, it requires consistent work.
I find that flexibility and creativity are closely related. Seeing possibilities and making connections requires a flexible mind and a willingness to explore. Creativity can be developed over time, but you must expose yourself to new experiences, media, and sometimes new people in order to get there. This new foundation of experiences will become a well that you can draw from when you are looking for inspiration or new material for a project.
Connecting the dots for people may appear as a kind of creative act, but it's really just a matter of taking a broader perspective. When you can see more, connections tend to become more apparent. Taking that step back to see the bigger picture is the what takes flexibility--and that flexibility is what enables you to be creative.