Most won't follow this advice, and will goop the gaskets with RTV. This is my method, and mine has "NEVER" had a leaky valve cover.
Clean the valve covers really well, attach cork gaskets to the valve covers with contact cement or 3M weather strip adhesive.
Apply a light coat of motor oil to the head side of the gasket. Since the head has a machined surface, it's perfectly flat, and needs no sealant.
Done like this and you won't have leaks and you can also reuse the gasket.