How can I make a precise one-way clutch using Technic elements? I need a mechanism so that if a (NXT) motor rotates forward, it moves one gear, but the other stays still, but if it rotates backwards, the first gear does not move, but the second one does. 

I found an example solution for one half of the problem here (I would need 2 of these), but that seems like it would take part of a rotation to pick up, and I need mine to be really precise.

I am building this for the Z-Axis/Extruder of the NXTStrap.
Would this solve your problem? Depending on the direction of the input, only one axle will turn.