What he said is mostly good.... but I would not stretch to the point where it "hurt like hell".
I spent almost two years trying to get flexible, I would stretch out for half an hour every time before I trained. Then, someone told me to stretch AFTER I trained. Bam, my flexibility went nuts. I went from round-housing rib cages to roundhousing the head of my 6'3" friend.
You should still stretch/warm up before training to prevent an injury... but the stretch AFTER you work out is where you make gains.
Defecato, ergo sum.
Leading the Thiago Silva bandwagon