Several threads over in Practical Travel Safety Issues with lots of detail.

The nutshell version: eligibility comes via a Trusted Traveler program (Global Entry) or from an offer from a participating airline, usually given to elites.

However, eligibility does not mean chosen for precheck every flight. There is supposed to be some element of randomness.

My personal experience is, as a GE member, I am 100% with precheck. Anecdotal data from the other forum indicate access as an elite is less than 100%.