What has worked for me consistently is removing the valve covers, clean the mating surfaces on the engine and covers with lacquer thinner and let dry.

Clean them again with lacquer thinner and wipe with a clean paper towel,let dry.

Purchase new cork valve cover gaskets (not rubber)
and apply hi temp silicone on the valve covers in a small bead line in the center of the gasket area.

Place the new gaskets in the cover & make sure the gasket "tabs" lay in the valve cover tabs as well.(If you have covers that do not have locating tabs in the cover,then trim off the gasket to match).

After you lay the gasket into the bead of hi temp, Carefully turn the cover over and lay down on a table or work bench. I add some weight to the top of the covers so the gasket is compressed in the hi temp and let dry overnight.

After the covers and gaskets have set over night, apply hi temp silicone or Permatex soft set to the engine cylinder head mating surface. Take the valve covers and install on each side of the engine one at a time cross torquing the bolts for each side. Let them dry overnight.

The next day, start your engine up and enjoy leak proof valve covers. Works for me every time.
HR 20