I replaced my passenger side valve cover gasket recently. No amount of tightening would have stopped this leak. BTW, I was able to remove and replace the cover simply by removing the electrical connectors from their mounting points and disconnecting the the two hoses.
Fairly common problem and pretty easy to fix (VERY easy to fix the passenger side). The driver side is a little more involved because you have to remove the upper intake manifold, but it's overall an easy DIY repair.