extreme cleanliness, make sure ALL surfaces are VERY clean, after scraping off the old gasket material, take a rag, spray some brake cleaner on it, then wipe down the head surface, so to rid it of any oil coating...
Take the valve covers, scrape them clean as well, and also use the brake cleaner to clean the surface of any oil coating,
use a very thin coat of high temp black RTV on the valve cover only, place the gasket on the valve cover only, place it centered with all holes, set it on the gasket surface on the ground for awhile until the gasket sets up so it won't move while putting it on the head....
once it's set up, install them on the head, no RTV on that surface, the gasket will seat to the surface when it heats up.
I do this even with chrome valve covers, and don't have any issues with leakage, and most chrome covers are hard to get sealed.
if you don't have them, try using the long spreader bars to assist with dispersing the torque of the bolts over the valve cover edges.
