I found cleanliness is next to godliness when I did mine ... errrr... well the SECOND time I did it, well...errr...the THIRD time.

First time I failed to carefully scrape off all the crap and corruption from the mating surfaces before installing the new gasket. Second time I managed to pinch a wire on the firewall side of the valve cover where I couldn't see it. Third time I learned to be patient and take my time. Three times the charm I guess...hope yours is sooner!