I've got a little op-amp based wind sensor board that I burned for my students. It works fine with just a puff of air. It's not so great at getting normal breaths (too slow), but it doesn't sound like that's what you're after.
It works like this, two temperature sensors sit on top of two surface mount resistors, which act as heaters. One temp sensor-heater is covered, the other is exposed. An op-amp amplifies the difference between the two sensors while cancelling out the common mode (absolute) reading. When you blow on the exposed sensor, it is briefly cooled off, and the signal is raised. The raised signal is amplified and fed through the exposed sensor's resistor, which heats up the sensor and cancels the signal. The response to getting puffed on, takes about .25 to .5 seconds and then about another second or two for the signal to go low again - read it with an A/D in. The parts are about $4-5 in small quantities.
I'll eventually release it as a kit but it's way on the back burner.
email if you want a board and we'll work something out - I burned quite a few.
pbadger AT verizon DOTTY net