Sobre la marcha....

How do you say....sobre la marcha?

We can use this expression "To play it by ear"
Example: Where are we going to have lunch tomorrow?
               mmmm...I don't know...We'll play it by ear!

Another useful and similar expression is " To cross that bridge when one comes to it "
We normally use this expression when dealing with a situation or problem only when it arises.
Example: What if you don't pass the exam?
               I haven't thought about that. I will cross that bridge when I come to it.