think outside the box

also think out of the box

to develop ideas that are different and unusual. 

• We need to think outside the box if we are going to come up with something really new.
• These guys are incredibly creative - they really know how to think out of the box.
  

Usage notes: sometimes used with verbs other than think: 

• You need to look outside the box and see what you can come up with.
  

Etymology: based on the idea that limiting your thoughts is like thinking inside a box which can contain only a certain number of ideas.

... http://idioms.thefreedictionary.com/box