In my first cultural victory, I had only one religion. I founded zero, one spread to me, and I spammed it to my cities and got a cathedral in each. I had a GPF that generated 14 or 15 GA's. The rest of the culture came from the 15+ towns in each of the three cities when I turned the slider to 90/100%. Free speech also helped.
This was played with victoria though, which is about as easy as a cultural victory gets.
Anyway, that said, I think cottages and peace (nice neighbors) are the key to a cultural victory.