This guide is extremely helpful. I did the reflow twice, and it finally worked on the 2nd try.
Make sure you heat the board components well to get the solder flowing. Also, make sure you let the board cool for 15-20 mins afterward.
Also, be careful to attach all the wires correctly, making sure they're all properly seated. If you mess up one connection, you'll have to do this time consuming process twice.
I also took the hard drive out, put it in a Ziploc bag, and placed it in the freezer the second time I did the reflow. The old "Freezer trick" is one I've seen save many hard drives, especially those of the Seagate variety in the PS3.
I'm not sure what made it work the second time, but be patient. It's worth trying more than once, so make sure to save some thermal paste.
I have the 80GB launch version of the PS3, btw, which is notorious for YLOD. My son plays his PS3 pretty hard, so I'm surprised it didn't break sooner.
Just be patient and take your time. It will work.