Some airlines/hotel/credit card programs provide it free for their highest elite customers but either way you would need to go for an interview...
There are many discussions about TSA Pre-check throughout FT but most are in sub-forums for the specific airline...
http://www.flyertalk.com/forum/ameri...solidated.html
http://www.flyertalk.com/forum/unite...ravellers.html
http://www.flyertalk.com/forum/delta...pre-check.html
http://www.flyertalk.com/forum/check...bal-entry.html