Damn header leaks! wtd?

I've always used Felpro with good results. As was already stated, make sure that you tighten down evenly.
One thing that I have found that was VERY important IMHO, was to fire up the motor and let it warm a little, shut it down and then re-tighten the header bolts. It is amazing how loose they become once they reach operating temp. By doing this re-tighten when hot method, I have yet to one leak.