Sheared a bolt.

[darrsi]

One method worth trying is welding a nut on the sheared bolt.
Getting steel (or stainless) screws out of ally is always a ball-ache anyhow, plus they they're Loctited in.
It's a galvanic reaction between two dissimilar materials that cause the seizure.

One common error when tightening bolts is to use ratchets etc that are far bigger than need be for the job in hand (compare an Allen key with a socket ratchet / bar for length and you'll see what I mean) - even a long Allen key can apply far more force than is often realised.

I'd imagine anyone who doesn't use a torque wrench will over tighten the majority of bolts on a bike.
The torque setting on disc bolts feels very light when it hits the clicking point, and you almost feel like giving it that little bit extra.
Add the fact that the heads of the bolts seem to be made from marshmallows and you can quite easily turn a very simple job into nightmare.  :'(