Be sure the spiral separator isn't clogged. Indeed, it's best to simply replace these every time the valve covers are removed.
If you replace it, though, be very careful. It's likely to break. Get the inside of the cover clean and dry enough so any broken pieces don't fall and stick in there, and can be removed with a vacuum.
As for the gaskets, go genuine and fit them precisely as noted in the manual. Thoroughly clean all the mating surfaces before installation. It should all look brand spanking new.
When cleaning them, avoid overly aggressive procedures. When using a scraper, go for a nylon model if possible. Use nylon brushes, as well. Any cleaning fluids should be carefully chosen. Some of them (including popular degreasers) will eat a solid millimeter of cadmium or nickle plating and pit the hell out of an aluminum wheel. You don't want something like that to happen to your aluminum.
Those are the kind of mistakes that lead to seemingly incurable leaks.
It's going to take a while, but we all have wasted time that can be put to better use.
Valve covers that aren't plain old stamped steel models can be tricky, as once the seeping starts, it's very difficult to completely eliminate.
Cheers,
Kennith
