One way valve with a flap.

thingiverse

In an update, I discovered that using nylon resulted in improved functionality; the flap effectively blocked 80-90% of fluid flow in reverse direction. The design allows for a single-piece print creating a one-way valve. Although not stellar, it works well, as blowing in reverse closes the flap while blowing forward opens it. Further details can be found at http://mymakerblog.wordpress.com/2013/05/31/fluidic-diodetrials-and-tribulations/.