One way valve with a flap.

Update:I printed this with nylon and the flap worked much better, blocking 80-90% of the fluid flow in the reverse direction!

Prints in one piece one way valve. Works, but not steller.

Blow in the reverse direction and the flap closes. Blow in the forward direction and the flap opens.

More here:
http://mymakerblog.wordpress.com/2013/05/31/fluidic-diodetrials-and-tribulations/