How can you make an fly effect like in flyff?

I want an sort of effect like in flyff that when u press a button u can fly freely not automaticly fly from A to B but like set gravity to like 0 and still u can like "walk" in the air?

Thx for helping if your trying and for viewing my question so u might think of a solution for it while doing something else!

Try this out:

http://www.unifycommunity.com/wiki/index.php?title=FlyThrough