How do pilots stay disciplined and use checklists, even after they have become routine?
You stay vigilant by having seen things go wrong.

Like Jamiec mentioned the importance of a proper preflight is drilled into you by your primary instructor from day one in light aircraft, and that mentality carries through all the way up to heavy transport-category aircraft: You want to find any problems you can while you're on the ground, because if you take a problem into the air with you it's a decision you can quickly come to regret.

Personally I've found all sorts of "interesting" things on preflight inspections of light aircraft, including (but not limited to):


Mud wasps building a nest in the pitot tube
Mud wasps building a nest in the fuel drain
(still don't know what was up with that)
Missing screws from a fairing
Flat-spot on a tire
A tennis ball stuffed in between two cylinders
(a common flight instructor trick to see if students are doing a thorough preflight)
Position light lenses installed backwards
(another common CFI trick, but this one was actually a maintenance shop screw-up)
Various light bulbs burned out
Water in the fuel after a heavy storm.


I've also heard some scary stories:


Migrating colony of bees hanging out on the tail of the aircraft
(described as "The tail was literally dripping bees")
Flight controls rigged backwards
Big gouge in the wingtip
(acquired by the previous pilot scraping the tip against another plane's tail taxiing in on the ramp)
The ever-classic "ran out of fuel because you didn't visually check the tanks"